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3DCITYGH
Federico Mario La Russa
An Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
University of Catania
Department of Civil Engineering and Architecture
PhD Course in Evaluation and Mitigation of Urban and Land Risks
XXXV Cycle
PhD Candidate
Eng. Arch. Federico Mario La Russa
PhD Course Coordinator
Prof. Eng. Antonino Cancelliere
Supervisor
Prof. Eng. Cettina Santagati
Co-Supervisors
Prof. Arch. Mariateresa Galizia
Prof. Eng. Ivo Caliò
Eng. Marco Intelisano
PhD Thesis Title:
3DCITYGH: an Expeditious Parametric Approach for Digital Urban
Survey and City Information Modeling of city-block Structural Models
To Catania, Florence and London,
the cities that made me an engineer and architect.
4
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
5
“The mind is not a vessel to be lled
but a re to be kindled.”
Plutarch
ABSTRACT
The aim of the thesis is the denition of a parametric modelling methodology
that allows, in a short time and at a sustainable cost, the digital acquisition,
modelling and analysis of urban aggregates with the aim of facilitating sei-
smic vulnerability mapping actions in historic centres. The research involves
the use of direct data (site surveys) and derived data (available geodata) for
the realisation of a parametric City Information Model (CIM). Grasshopper, a
Visual Programming Language (VPL) within Rhinoceros, already widely used
in the scientic community, was chosen as the working environment for the
parametric computational design.
The methodology consists of several progressive steps that can be pursued
through two innovative methods (Survey-to-CIM and Scan-to-CIM) dened
and developed in this thesis. The choice of which method to adopt depends
on the availability of project resources and the type of urban centre being stu-
died. The rst method, Survey-to-CIM, is intended to be a low-cost solution
that integrates dierent data and acquisition techniques (direct survey, pho-
to-rectication, expeditious 360 photogrammetry, Street-Level imagery, etc.)
within a parametric and responsive ow useful for smaller urban centres and
without the need for special professional skills. The second method, Scan-to-
CIM, is developed to automate the cognitive operations of interpretation and
input of survey data performed by the surveyor. This task is carried out by an
Articial Intelligence system that uses Machine Learning techniques (in parti-
cular Random Forest) to identify remarkable geometric-architectural features
(openings, walls, stringcourses) within point clouds obtained through digital
surveying. Both methods lead to the denition of a parametric CIM system
developed in VPL.
The generated CIM model is equivalent to the denition of a semantic 3D City
Model since all generated geometries, from the envelope to the interior, adhe-
re to a semantic structure that denes relationships and dependencies. Since
there are no guidelines in literature regarding the semantic structuring of city
models in a parametric environment, an innovative format dened CityGH is
proposed in this thesis. Based on the CityJSON standard, it proposes the
same semantic structure of CityJSON but adapting it to the Data Structure of
Grasshopper (Data Trees). This format is therefore proposed as an interchan-
ge format within parametric models to facilitate their dissemination and appli-
cation. Thanks to its semantic structure, the parametric CIM model is capable
of storing attributes and metadata that are fundamental steps in facilitating
the beginning of the FEM analysis activities.
The parametric CIM model dened so far also allows the extraction of structu-
ral geometric models, both NURBS and MESH, necessary for the execution
of FEM analyses. In particular, a workow was developed in the thesis that
allows FEM analysis both within the VPL environment (Karamba3D, Alpaca4D
- OpenSees plugins) and in external software dedicated to structural analysis
(SAP2000, FEM-Design).
• City Information Modeling;
• 3D City Models,
• Computational Design;
• Parametric Modelling;
• VPL;
• Digital Survey,
• Urban Survey,
• Seismic Risk,
• Scan-to-FEM
KEYWORDS
SOMMARIO
Scopo della tesi è la denizione di una metodologia di modellazione parametri-
ca che consenta, in tempi brevi e con costi sostenibili, l'acquisizione digitale, la
modellazione e l'analisi di aggregati urbani con l'obiettivo di facilitare le azioni
di mappatura della vulnerabilità sismica nei centri storici. La ricerca prevede
l'utilizzo di dati diretti (rilievi in sito) e derivati (geodati disponibili) per la realizza-
zione di un City Information Model (CIM) parametrico. Come ambiente di lavoro
per la progettazione computazionale parametrica è stato scelto Grasshopper,
un Linguaggio di Programmazione Visuale (VPL) all'interno di Rhinoceros, già
ampiamente utilizzato nella comunità scientica.
La metodologia è composta da diverse fasi progressive che possono esse-
re perseguite attraverso due metodi innovativi (Survey-to-CIM e Scan-to-CIM)
deniti e sviluppati in questa tesi. La scelta di quale metodo da adottare di-
pende dalla disponibilità di risorse del progetto e dal tipo di centro urbano og-
getto di studio. Il primo metodo, Survey-to-CIM, mira ad essere una soluzione
low-cost che integra diversi dati e tecniche di acquisizione (rilievo diretto, foto
raddrizzamenti, fotogrammetria 360 speditiva, Street-Level imagery, etc.) all’in-
terno di un usso parametrico e responsivo utile per centri urbani minori senza
la necessità di particolari competenze professionali. Il secondo metodo, Scan-
to-CIM, è sviluppato per automatizzare le operazioni cognitive di interpretazio-
ne ed inserimento dei dati di rilievo eseguite dal rilevatore. Questo compito è
svolto da un sistema di sistema di Intelligenza Articiale che utilizza tecniche di
Machine Learning (in particolare Random Forest) per individuare caratteristiche
geometriche-architettoniche notevoli (aperture, muri, marcapiani) all’interno di
nuvole di punti ottenute tramite rilievo digitale. Entrambi i metodi portano alla
denizione di un sistema CIM parametrico sviluppato in VPL.
Il modello CIM generato corrisponde alla denizione di un 3D City Model se-
mantico in quanto tutte le geometrie generate, dall’involucro agli interni, rispon-
dono ad una struttura semantica che denisce rapporti e relazioni. Non essen-
do presente in letteratura alcuna linea guida circa la strutturazione semantica
dei modelli di città in ambiente parametrico, in questa tesi viene proposto un
formato innovativo denito CityGH, che su riferimento dello standard CityJ-
SON propone la medesima struttura semantica di CityJSON ma adattandola
alla Data Structure di Grasshopper (Data Trees). Tale formato si propone quindi
come formato di interscambio all’interno di modelli parametrici per facilitare la
diusione e l’applicazione degli stessi. Grazie alla struttura semantica, il model-
lo CIM parametrico è capace di immagazzinare attributi e metadati che risulta-
no passi fondamentali per facilitare l’inizio dell’attività di analisi FEM.
Il modello CIM parametrico n qui denito consente anche l’estrazione dei
modelli geometrici strutturali, sia NURBS che MESH, necessari per l’esecu-
zione delle analisi FEM. In particolare, nella tesi è stato sviluppato un usso
di lavoro che consente l’analisi FEM sia all’interno dell’ambiente VPL (plugin
Karamba3D, Alpaca4D - OpenSees) che in software esterni dedicati all’analisi
strutturale (SAP2000, FEM-Design).
PAROLE CHIAVE
• City Information Modeling;
• 3D City Models,
• Computational Design;
• Modellazione Parametrica;
• VPL;
• Rilievo Digitale,
• Rilievo Urbano,
• Rischio Sismico,
• Scan-to-FEM
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
10
INDEX 01.
02.
Introduction
1.1 Research Context
1.2 Motivation for the study
1.3 Research Goals (Aim and Objectives)
1.3.1 Aim of the study
1.3.2 Research Questions
1.3.3 Objectives
1.4 Methodology proposal
1.5 Signicance of the study
1.5.1 Theoretical
1.5.2 Practical
1.6 Experimental context and case studies
1.7 Thesis Structure
Background
2.1 Digital Urban Survey: evolution and techniques
2.1.1 An overview of digital surveying technologies and approaches
2.1.2 Articial Intelligence for Cloud Segmentation and Classication
2.2 Parametric Modelling
2.2.1 Implicit and Explicit Parametric Modeling
2.2.2 Visual Programming Languages
2.2.3 Grasshopper for BIM, Geomatics, Urban Modelling and FEA
2.3 Urban Seismic Risk Assessment: the Southern Italy scenario
2.3.1 School of thoughts: statistical, mechanistic, holistic
2.3.2 Previous studies on seismic vulnerability for the city of Catania
2.3.3 Cloud-to-FEM methods: BIM and NURBS modeling
2.3.4 Parametric and VPL based related works on structural analysis
2.4 From 3D City Models to City Information Modeling
2.4.1 Denition, standards and applications of 3D City Models
2.4.2 City Information Modeling: beyond GIS and BIM
2.4.3 Relevant Case Studies
2.5 Conclusions and research challenges
16
17
22
27
1
1
1
28
31
1
1
33
34
40
42
2
2
57
2
2
2
66
2
2
2
2
77
2
2
2
95
01.
02.
Methodology
3.1 3DCityGH: a parametric CIM-based methodology for
3D City Models
3.1.1 Survey-to-CIM
3.1.2 Scan-to-CIM AI-based workow
3.1.3 CIM-to-FEM
3.2 Experimental context and choice of case studies
Case studies
4.1 Minor urban center scenario: the territorial scale
4.1.1 An Anti-Fragile City Information Model for Fleri
4.2 Major urban center scenario: the district scale
4.2.1 Comparison of Digital Urban survey terrestrial techniques
4.2.2 Cityblock Survey-to-CIM application with FEM models
4.2.2 Cityblock Scan-to-CIM with AI Segmentation
4.3 A parametric 3D city models format: CityGH
Discussion of Results
5.1 Minor urban center scenario: Fleri
5.2 Digital Urban Survey Techniques
5.3 Survey-to-CIM-to-FEM: the casa terrana application
5.4 Scan-to-CIM: AI parametric workow in Catania
5.5 A parametric format for a parametric 3D City Model: CityGH
Conclusion and Future Work
110
113
3D
3
3
3
126
132
134
4
149
4
4
4
200
216
218
219
220
221
223
226
03.
04.
05.
06.
Appendix
Appendix 1 | Sinossi in lingua italiana
Appendix 2 | List of publications and awards
Acknowledgments
232
233
238
242
-
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14
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
15
01.
16
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
INTRODUCTION
17
1.1 Research Context
Earthquakes are natural phenomena that testify the dynamic activity of the
planet in which we live. Their activity over the millennia has inuenced the
dynamics of entire civilizations always pushing for greater progress in con-
struction and architecture because of the need to survive calamitous events.
Seismic risk is a present-day subject for many countries around the world.
Despite the knowledge and methods we currently have for earthquake coun-
termeasures, economics has always been a barrier to the establishment of
a safe state in many at-risk nations. As matter of fact, engineering seismic
risk mitigation actions involve a knowledge phase that has costs even before
those related to the necessary structural interventions.
In this scenario, knowing the dierent degrees of seismic vulnerability of an
urban area makes it possible to establish priorities for intervention in order to
optimize the nancial resources available for the communities. For seismic
risk mitigation, it is necessary to operate in terms of prevention and to propo-
se plans that aim at the progressive reduction of exposure where economic
capacity does not allow for intervention in the short term. Risk assessment is
fundamental to trigger anti-seismic improvement processes based on inter-
vention planning, especially with reference to historical city centers that were
constructed decades (and centuries) before any regulation regarding seismic
safety. Therefore, seismic vulnerability mapping is needed to estimate the
level of risk, taking advantage of databases containing elementary and easily
available information (number of oors, age of construction, materials, etc.)
even at the scale of the individual building, in the unicum that it represents.
Among the areas most at risk there is the Mediterranean one, in which Italy is
one of the countries at a very high level of risk. Indeed, Italy is located at the
meeting point of the Eurasian plate and the African plate, in this area there
are numerous faults, which determine the geological instability of the territory.
Taking into account the earthquakes that have occurred in the past and con-
tinue to occur, the Italian peninsula appears to be particularly aected. More
than thirty very strong earthquakes (with a magnitude greater than 5.8) have
occurred since 1900 (Milano, 2013).
The vast extent of the historic (currently inhabited) building heritage and the
large number of monumental architectures distributed all over the country
made institutions dene methodologies and approaches for the analysis of
urban areas and valuable architectures. In 2011, the Italian Ministry of Infra-
structure and Transport approved the “Guidelines for Seismic Risk Asses-
sment in Cultural Heritage” (which in this thesis will be referred to as LLGG
2011) in which a workow is dened for the knowledge of the urban built
environment functional to seismic safety analysis (Ministry of Culture, 2018).
The geometric survey and analysis of the historically-built masonry building,
its technological system, and the construction techniques that distinguish it,
are fundamental steps in analyzing and assessing seismic safety. Geometri-
cal survey and technological recognition on a large scale (also by means of a
01.INTRODUCTION
18
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
rigorous classication of building elements) therefore become the main tools
to be investigated and used for the purposes of mapping vulnerabilities as
closely as possible to the actual state.
Therefore, being able to store and process in the same three-dimensional
model the data relating to the registry of each building, its technical-con-
structive system, and its state of conservation represents today, more than
ever, an invaluable resource to be able to estimate, quickly and with accep-
table margins of error for an urban scale assessment, seismic vulnerability,
possible safety interventions and the priority of interventions for the recovery
of entire urban city blocks of considerable cultural importance. Currently, the-
re might already be numerous data useful for designing a model such as the
one described. Indeed, there are several (international, national, and local)
databases and previous research that have not yet been systematically orga-
nized in a comprehensive city-wide analysis process.
Nowadays, many disciplines are experiencing a transformation process
through complex digital media as part of the fourth industrial revolution (Indu-
stry 4.0), which is envisioned as a synergy of processes (automated and not)
amongst diverse sectors. This transition has an impact on the construction,
architectural, and spatial planning industries as well. These industries are
transforming their working methods in favor of a digital strategy that opti-
mizes processes by cutting costs and time while boosting the quantity and
quality of output.
In this direction, one of the tools used by Civil Protection1 to estimate the
damage and viability of buildings in the post-earthquake phase is the manual
compilation of Aedes matrix-based templates (g. 1). Over the years, these
have been complemented by the Erikus digital information system (Regione
Piemonte, 2018). This digital system makes it possible to put together data
from dierent sources (such as previous survey reports) and semi-automati-
cally compile it within a Geographic Information System (GIS) environment
Aedes forms. Although in several public administrations a process of digiti-
zation of the data present in the Aedes cards has been undertaken, these are
delivered through GIS software which hardly manages to eectively represent
data and features at the architectural scale.
GIS and BIM (Building Information Modeling) methodologies allow for the
consideration of the territorial scale (GIS) to the individual building organi-
sm (BIM) in relation to specic and multipurpose data management activities
(g. 2). These methodologies are well suited to the needs imposed by the
activities required for seismic risk assessment. Unfortunately, they are often
adopted separately compared to the required level of knowledge, which le-
ads to a wider approximation for the urban scale, aecting insucient detail
to establish the level of seismic safety with scientic validity.
1 Civil Protection is an italian national institution for the safety of the citizens against any kind of risk
scenario.
19
Fig. 1 | Templates used for pre-
venting and managing activities
related to seismic risk scenarios
(from left to right: GNDT,
AeDES, SIVARS). These forms
are filled out manually by an
operator following predefined
procedures. Thus, the validity
of these forms depends on the
competence of the operator.
Fig. 2 | Conceptual diagram
of the workflows related to the
knowledge process described
in LLGG 2011.
Over the past few decades, the human-machine relationship has progressed
signicantly due to the evolution and deployment of increasingly advanced
technologies. One of the main objectives in the AEC sector has become to
develop semi-automated solutions and workows that can minimize repeti-
tive and time-consuming activities, thus allowing professionals to focus on
more valuable and relevant tasks. This has also had repercussions in the
technological advancement of the previously mentioned methodologies by
providing a range of solutions useful for dierent scenarios: from environmen-
tal analysis to form optimization and risk analysis.
In the context of this range of methodological paradigms and innovative so-
lutions, the need has arisen in the national and international research com-
munity for a new approach that allows the integration of data from GIS and
BIM environments with the aim of developing monitoring, management, and
intervention strategies on an urban scale with a holistic approach: the so-cal-
led City Information Modeling (CIM). Academic contributions in this eld are
copious, spread all over the world, and have a strongly interdisciplinary cha-
racter. CIM is an urban-scale Information Model in which features common
to GIS and BIM models converge, from small to large scales. It is therefore
01.INTRODUCTION
20
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
a methodology that enables better structuring, processing, integration, and
management of data and consequently multi-criteria queries and analyses.
In many cases, the logic used to structure CIMs is algorithmic, utilizing pro-
gramming environments that permit the generation of codes that directly
work in the 3D modeling space and construct parametric and responsive
models for very specic objectives. There are currently few applications in the
eld, and those that exist have been created mostly in academic and softwa-
re-related elds where it is simpler (for a general domain expert) to acquire
computational skills and interact with other programmers. This is due to the
high-level programming abilities required for these projects.
However, the gap between ‘designer’ and ‘programmer’ has been conside-
rably reduced with the introduction of Visual Program Languages (VPLs) wi-
thin modeling software to develop computational codes. Their ease of use
lies in their visual nature and in a vocabulary of ‘components’ where the main
grammatical rule consists in the relationship between input and output. The
exibility of VPLs makes them powerful tools in the management and proces-
sing of data (solving data interoperability issues between dierent BIM and
GIS software, processing data of dierent natures), in the combination of data
and geometric-spatial information, and in the use of complex computational
techniques ranging from evolutionary optimization to Articial Intelligence ap-
plications. In addition, the use of VPL can in fact smooth the learning curve
required to adopt this solution thus supporting the development of increa-
singly advanced digital solutions used by a wider spectrum of professionals
(g. 3).
Fig. 3 | Conceptual scheme for
City Information Modeling using
VPL programming environmen-
ts related to the AEC industry.
A parametric and responsive CIM can form the core of a possible DSS (Deci-
sion Support System), i.e. a digital support system for all those who need to
undertake strategic decisions or automate manual operations so that profes-
sionals are allowed to concentrate on the tasks that need more of their focus.
These systems make it possible to increase the eectiveness of analysis by
21
overcoming the limitations of operational research and quickly extracting all
useful information from large amounts of data. Moreover, due to its parame-
tric and responsive nature, the structure of a CIM model can be easily exten-
ded, accommodating new instances that will become new parameters, and
re-adapted to other study contexts/scenarios.
Aim of the this work is the denition of a parametric modeling methodology
that allows, in a short time and with sustainable costs, the digital acquisition,
modeling, and analysis of urban aggregates with the aim to facilitate actions
of seismic vulnerability mapping in historical centers.
In particular, this research focuses on how the disciplines of architectural re-
presentation and surveying, in their digital applications, can accelerate the
preliminary knowledge phase functional to seismic safety analysis in this way
reducing the work burden on structural engineers. The main topic covered
is 3D city modeling, seeking its contextualization with reference to the cur-
rent City Information Modeling methodology approach and with application
toward the creation of geometric models for FEM analysis. The application
elds in which the work lies are those of digital surveying and explicit parame-
tric modeling through the use of VPLs as programming tools. With reference
to these elds, this work digresses with regard to current urban modeling
procedures with an in-depth look at Scan-to-FEM techniques that enable di-
scretized geometric models at the urban scale to conduct structural analysis.
01.INTRODUCTION
22
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
1.2 Motivation for the study
The high seismicity of the Italian territory is undermining the integrity of our
cities' built heritage. The workow indicated by the LLGG 2011 Guidelines for
seismic risk assessment and reduction describes an analysis of individual bu-
ildings. However, its extension to the scale of the city and urban aggregates
requires that the survey takes into account the complexity and multidimen-
sionality of the dierent aspects that characterize the city and urban structu-
re. In fact, every single unit, besides being characterized by its specicity, is
also related to the surrounding buildings. The consolidated methodologies of
the Urban Survey (Baculo, 1994; Vernizzi, 2006; Caniggia et al., 2008; Garzi-
no et al., 2019, Boido et al., 2021), describe and represent "the set of values
present in the investigated realities, in order to build the set of formal and
structural invariants" (Coppo and Boido 2010, p. 12), are still very topical
today (g. 4).
Fig. 4 | Survey of the neigh-
borhood of Via Pietro Micca
(Turin) and analysis of the built
environment according to UNI
7310/74 (Coppo & Boido,
2010).
They provide the scientic background to the experiments related to the ap-
plication of the City Information Modeling paradigm for the development of
informed and responsive city models for seismic vulnerability assessment.
According to ISTAT data, more than half the population in Italy lives in buildin-
gs constructed before the 1970s. Moreover, the entire national territory (with
the exception of Sardinia at present) is completely exposed to seismic risk.
These indicative data demand attention since the rst mandatory seismic
standards for the design of residential buildings date back to the 1980s. The-
re is therefore a need to adapt and improve the performance of the existing
building stock in relation to seismic events.
23
However, economic resources are limited compared to the extent of the cur-
rent building heritage, so it is essential to establish priorities for intervention
(both public and private). This scenario is especially true in small realities,
the so-called minor urban centers, which unfortunately when hit by seismic
events are particularly vulnerable and for this reason are often the object of a
gradual and often irreversible depopulation process, making the vast urban
and architectural heritage represented by historic centers even more fragile.
A number of works in the literature highlight the eorts of the scientic com-
munity to manage the complexity of urban space in all its dimensions (phy-
sical, economic, social, risks, etc.). The smart cities paradigm for enhanced
management of all urban processes is becoming an increasingly widespread
practice but is almost always relegated to large urban centers with the sup-
port of considerable economic resources.
In reference to seismic vulnerability in terms of SLV (Lifesaving Limit State),
as described in the LLGG 2011, it is possible to determine it, with a level
of accuracy suitable for urban planning needs, by means of methodologies
applicable on an urban scale. “The assessment of the SLV (Lifesaving Limit
State) using simplied methods based on a limited number of geometric and
mechanical parameters or using qualitative data (visual interrogation, reading
of building characters, critical and stratigraphic survey)” - LLGG 2011. These
methods are widely used for LV1 assessments (“for seismic safety asses-
sments to be carried out at a territorial scale on all protected cultural heritage
assets” - LLGG 2011) while LV2 and LV3 levels (which represent “an accurate
assessment of the seismic safety of the artifact”) are the objects of greatest
interest and are obtained through modeling procedures that are onerous in
terms of time, resources and skills employed.
In addition, the inability to determine certain building elements of the con-
truction, its geometric characteristics, or construction techniques during the
expeditious survey phase, invalidates the reliability of these models at all
three levels of evaluation, further diluting the analysis time.
These considerations lead to reections on how to facilitate the knowledge
process as set out in the LLGG 2011 in a sustainable and accessible way,
even for smaller realities. Getting from survey to data and from these to sei-
smic assessment as quickly as possible can be the key to seismic prevention
in the territory.
Statistical methods and damage probability matrices are currently used
to facilitate seismic safety knowledge and assessment operations. These
methods, despite being fast and low-cost, often return results that dier from
reality and are prone to the expertise of the operator. Indeed, in order to have
more accurate information, it is necessary to conduct Finite Element Analysis
(FEA) in digital environments. However, this type of analysis requires consi-
01.INTRODUCTION
24
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
2 Over the past few years, great progress has been made in semantic segmentation through the appli-
cation of Artificial Intelligence (Grilli et al., 2021).
derable surveying and modeling time and therefore is hardly applied to the
urban scale. The key to implementing this analysis at the urban scale lies in
the way of acquisition of urban data (geometric and informative) and in their
management within appropriate modeling environments that allow their tre-
atment. In many regions exposed to seismic risk, the data needed to perform
this type of analysis are often out of date, unusable, or absent (Geiß, 2015).
Furthermore, traditional processes for acquiring this information, which inclu-
des building-by-building inspections, are costly and time-consuming, making
them impractical for evaluating a large building stock (Sulzer et al., 2018;
Greco et al., 2020).
On the other hand, thanks to the evolution of geomatics methodologies seve-
ral solutions are available today for acquiring large amounts of geometric data
and generating complex models of real-world structural systems by exploi-
ting point cloud processing. These procedures (known as Scan-to-FEM) are
of support to dene informative digital models useful for the purposes of
modeling for FEA analysis. Although point cloud classication is largely opti-
mized2, automatic transition to useful informative digital models (BIM/GIS) for
FEM analysis is currently a subject of research and in recent years parametric
modeling is almost mainly used to solve problems related to interoperability
with structural analysis software.
Compared to the research and mapping already carried out in the international
and national elds that have contributed to the creation of dierent types of
databases (WebGIS of territorial administrations, cadastral databases, open
source portals such as OpenStreetMap, NASA topographical data, research
on typical historical construction equipment, etc.) there are few experiments
oriented towards the systemization of the dierent data using sophisticated
and advanced techniques, through the automatic or semi-automatic imple-
mentation of three-dimensional systems at the scale of the city, up to the use
of Articial Intelligence to facilitate the seismic safety assessment of historical
city blocks.
OpenData represents a huge potential for applications in various sectors
and, in particular, in territorial government. To date, OpenStreetMap (OSM)
is the largest collection of open-licensed geospatial data. It is a collaborative
project started in 2004 to which one can contribute as Volunteer Geographic
Information (VGI). These data can be used to create and implement 3D city
models that represent Digital Twins (DT) of real cities. The concept of DT,
although born in the manufacturing industry, has become central in several
elds of knowledge, and represents a new way to better understand cities
and intervene in their future. In the conceptualization of DT, its eectiveness is
closely linked to the sensor network from which it acquires data (Batty, 2018).
Therefore, dealing with Big Data (conspicuous masses of digital data of dif-
ferent nature and sources) may contribute to the development of new appli-
25
cations, also considering the numerous databases of seismic vulnerability
assessments already drawn up in Italy. In this direction, possible experimen-
tation could be conducted by developing an innovative methodology using
a CIM-type Informed Model of the city that allows, in the same digital ecosy-
stem and overcoming interoperability issues, to process data typical of GIS
environments by relating them to three-dimensional informed models typical
of the BIM approach.
Indeed, considering the impact that BIM methodologies are having in the
construction sector (also due to the cogency of some recent regulations on
public procurement, D. Lgs. 50 of 2016 - Public Contracts Code and more
generally related to the digitization of the assets of public administrations and
the city) it is conceivable to assume that in the next few years there will be
an increasing number of public administrations that will have at their disposal
a large number of BIM models (as virtual replication of existing buildings se-
mantically enriched with data) even for small private buildings that constitute
a large part of minor and major historic urban centers.
In this scenario, City Information Models which rely on BIM and GIS assets
could represent the digital environment in which data useful for FEM analysis
can be created, treated, and exploited. The adoption of Computational De-
sign into the eld of 3D City Models led to the combining BIM and GIS pro-
cedures while minimizing input data and steps required for the main purpose
of the 3D city model requested. In addition, the granularity of the data allows
for a high level of interoperability among software environments involved in
these processes.
Taking the above into account, two particular research gaps are highlighted in
relation to the topic under consideration.
Currently, there are many 3D city model applications that integrate GIS and
BIM elements, often also making use of the interoperability potential of VPL
environments. However, frequently these applications result mostly in visuali-
zation with low levels of interaction (in some cases exclusively consultation of
the input material). There are very few examples in the national and internatio-
nal landscape of integrating geometry and data semantically in accordance
with the principles dened by City Information Modeling. This deciency can
be traced to the inobservance of international standards for 3D city modeling
such as those dened by OGC CityGML (OGC CityGML 3.0, 2022). This shor-
tcoming is very often due to the fact that these standards are tied to a digital
format (CityGML) that is dicult to replicate in the absence of high program-
ming skills. This leads to the absence of applications in the eld of parametric
modeling of informed city models adhering to the directions and standards
close to the CIM methodology.
01.INTRODUCTION
26
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Regarding the pipeline of work related to the creation of structural geometric
models based on reality capture strategies, it is also worth mentioning that
there are not many applications for the adoption of computational design to-
ols for optimizing and accelerating the Scan-to-FEM pipelines (Funari et al.,
2021). Another research gap can be found in the lack of applications between
city block acquisition modeling and a semi-automatic transition to FEA.
In conclusion, it is worth investigating a system that can accommodate these
models and extract from them valuable information for seismic risk asses-
sment and mitigation by also integrating material from expeditious surveys
conducted on-site (photogrammetry, laser scanners, visual surveys, non-de-
structive surveys, etc.). Therefore, it is useful to realize a CIM-type Informed
Model capable of gathering data from already existing databases as well as
on-site acquired/collected data. The CIM model thus dened in a VPL-based
parametric modeling environment can be appropriately tuned to obtain di-
scretized structural geometric models ready to be launched within structural
FEM software.
27
1.3 Research Goals
The theme of this thesis is the denition of a parametric modeling methodo-
logy that allows, in a short time and with sustainable costs, the digital acqui-
sition, modeling, and analysis of urban aggregates with the aim to facilitate
actions of seismic vulnerability mapping in minor and major historical urban
centers.
Hence, the Main Research Question (MRQ) has been dened:
- MRQ: through which strategy is it possible to speed up the knowledge
process established in the LLGG 2011 in a sustainable way that leads to
an assessment that is as accurate and adherent to reality as possible in
relation to the urban scale and the requirements of structural analysis?
Due to the complexity of the subject and the consequent need for an interdi-
sciplinary approach, it seemed eective to pursue this direction of research
by addressing three additional research questions (RQs) that are instrumental
in dening the research objectives:
• RQ 1: Is it possible to semi-automate the process of geometric know-
ledge on the urban and architectural scale of the built heritage in histo-
rical centers?
• RQ 2: Is it possible to conduct expeditious and sustainable digital ur-
ban survey campaigns?
• RQ 3: Is it possible to obtain structural geometric models from a City
Information Model to speed up the execution of structural analyses?
In connection with the above-mentioned RQs, it is possible to dene the Aim
(A) and Objectives (O) of the research as follows below:
• A: To develop an accessible, semi-automatic, and expeditious proce-
dure to support seismic assessment procedures of historic urban cen-
ters and fabrics according to mechanistic methods.
- O1: To develop a procedure for the investigation, survey, and para-
metric urban modeling of the existing heritage (specically residen-
tial buildings in historical urban city blocks).
- O2: Analysis and comparison of expeditious digital acquisition tech-
niques of urban canyons.
- O3: Discretization of parametric information CIM models into structu-
ral geometric models useful for generic FEM analysis engines.
01.INTRODUCTION
28
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
1.4 Methodology proposal
The methodology proposed in this research nds its background in the ARIM
methodology and the 2014 FIR project entitled “The Seismic Vulnerability
of Historic Aggregate Buildings. New structural modeling and expeditious
methodologies and approaches” (Calvano et al., 2019). This thesis advances
this methodology by proposing an integrated approach between the two di-
sciplines (urban surveying and parametric modeling, on the one hand, and
building science on the other) without solutions of continuity.
The research involves the use of direct data (site surveys) and derived data
(Geo-Data present in the territory) to create a City Information Model to sup-
port seismic assessment at the urban scale (g 5).
This can be done by means of a parametric modeling system designed taking
into account the possibility of using existing databases and data types that
can be derived from operational protocols already employed by professio-
nals. Two scenarios for the digital acquisition of urban canyons are conside-
red. These are meant to represent the scalability of the digitization process
and the dierent opportunities that small and large urban centers can adopt
in accordance with their resources.
The rst (Survey-to-CIM) takes into account current methods used for direct
expeditious surveys by professionals and implements their workow through
VPL code developed using a GIS approach and starting from open databases
such as OpenStreetMap. This approach is intended to be a low-cost solution
that does not involve the use of advanced technologies nor requires high le-
vels of expertise but signicantly improves the collection and organization of
data in order to inform the CIM model.
The second approach (Scan-to-CIM) adopts more advanced technologies
and techniques related to the automatic semantic classication of urban
canyon point clouds using Articial Intelligence techniques (Random Forest).
These semantic decomposed clouds are interpreted by the code developed
in this work and the result is the automatic modeling of the city block under
study. This approach, although a greater degree of automation is gained than
the previous one, requires more expertise (particularly in the rst phase of
classication with Random Forest) and possibly mid and high-end acquisition
sensors. However, in the long-term view, this approach will require less and
less eort on the part of the operator.
In both scenarios, the system allows the individual three-dimensional units
(the historic buildings of interest) to be enriched with information and geome-
tries, with dierent levels of reliability in relation to the source of the informa-
tion, and managing all single architecture components together by means of
semantic relationships as employed in BIM methodology.
29
Fig. 5 | Conceptual outline of
the proposed methodology.
01.INTRODUCTION
30
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
At this point, the Informed Model permits to automate the creation of the
structural model from the CIM model allowing a rapid generation of geome-
tric models for structural analysis with a high level of interoperability between
dierent structural analysis software.
Following is a summary description of the main phases and actions that con-
stitute the methodology adopted in this research:
Phase 1 | Data input and model at the territorial/urban scale
• Data collection, protocol and input data denition
• City model at the urban scale | LOD 0 - LOD 2
Phase 2 | Knowledge of the historic built environment
• Digital expeditious survey of the urban canyons
• Study of recurring building types
Phase 3 | Model at the architectural scale
• City model at the architectural scale | LOD 3 - LOD 4
Phase 4 | Semantic structuring and information enrichment
• Semantic structuring of the parametric model according to the prin-
ciples of the CityJSON standard (from the city block to the building
component)
• Information enrichment of individual objects through the assignment of
attributes collected during Phase 2
Phase 5 | Structural geometric model
• Discretization (Mesh) of the NURBS model for the purpose of structural
analysis
• Interoperability validation of the model
31
1.5 Signicance of the Study
1.5.1 Theoretical
Historical centers represent the memory of the choices that have drawn the
history of a place; that is, the outcome of transformations and stratications
of the cities across the centuries. The knowledge of a historical urban en-
vironment requires an analytical methodology articulated on several inter-
connected levels of investigation to model a multi-layered complexity that
encompasses the geometric and stylistic features of places (blocks irregula-
rities, narrow streets, stratied buildings), the accessibility (pedestrial zone,
no yzone), the use of existing data (GIS, cartographies).
Urban survey and 3D modeling play a pivotal role in the various phases of the
cognitive process. Indeed, the interpretation of the collected geometric-spa-
tial data together with the critical reading of historical documents (i. e. cada-
stral maps, archive sources) allows to understand the link between the traces
of the past and the present. Today the challenge for historical centers is dual:
on one side, to make use of digital technologies to acquire data, on the other
one to create systems that allow to manage, visualize, enquire and use (i. e.
for simulation purposes) these data in a unique digital ecosystem. Moreover,
the anthropic and natural hazards (earthquakes, oods, etc) to which histori-
cal centers are prone require a continuous monitoring of their state of conser-
vation, necessarily sustainable in time and resources.
The work aims to broaden the debate about digital urban surveying activities
by adding among the limitations to be considered the sustainability, timing
and eectiveness of the acquisitions conducted with a view to optimizing
operations and overall cost reduction against the entire stock of buildings to
be assessed.
In addition, this thesis seeks to open, particularly at the national level, a de-
bate about the conscious adoption of the City Information Modeling metho-
dology so as to facilitate, as was the case with BIM, both theoretical and te-
chnological and regulatory development towards holistic and interdisciplinary
solutions.
1.5.2 Potential impact
The VPL codes obtained in this thesis were conceived and developed as pro-
totype plug-ins that could be easily used (by experts and non-experts alike)
so as to maximize their benets in industry and research applications as well
as in the public sector.
Addressing the topic of City Information Modeling and current standards in
3D city modeling within a parametric environment provides a practical para-
digm to refer to for future developments in the eld of urban modeling using
parametric tools.
01.INTRODUCTION
32
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
In addition, the various critical issues encountered in the digital acquisition of
the urban scenarios of the case studies have led to the realization of analyses
and comparisons, both analytical and qualitative with respect to the objecti-
ve, which contribute to the international academic debate about urban sur-
vey operations in typical situations of historic city centers common to many
countries around the world.
The CIM models to be obtained present high levels of adaptability and re-
sponsiveness over time, thus becoming a powerful tool for knowledge, analy-
sis, and monitoring of the built heritage. Such models will also be able to ser-
ve as a system for storing data collected by any acquisition instruments (such
as accelerometers) including those already installed in some of the city’s hi-
storical buildings, thus also favoring the process of seismic micro zoning and
relating this data to the family of parameters (geometric and technological)
belonging to the individual building. This represents a step towards the tran-
sition from the traditional forms of urban planning to the new management of
cities according to the Smart Cities model with a view to holistic sustainabili-
ty, all through advanced strategies of expeditious urban digital surveying and
parametric modeling.
33
1.6 Experimental context and case studies
The present work focuses on the denition of VPL code that can elaborate
survey data of dierent kinds for the reconstruction of semantically rich 3D
urban landscapes and city blocks. The thesis and the experimental works are
developed within the Ph.D. Course in Evaluation and Mitigation of Urban and
Land Risk at the Department of Civil Engineering and Architecture (DICAr) of
the University of Catania (UniCT) from which the work is funded. The research
topics developed in the Ph.D. course are closely related to the territory of the
city of Catania, which is subject to multiple categories of risk (seismic, volca-
nic, hydrogeological, etc.). This territory, particularly fragile and at the same
time with a signicant architectural heritage, appears to be a benchmark for
other similar national and international scenarios (especially in the Mediterra-
nean context).
Specically, the thesis has been developed within the Laboratory of Archi-
tectural Photogrammetry and Surveying “Luigi Andreozzi” (DICAr) under the
supervision of Prof. Cettina Santagati (supervisor, Professor of Parametric
Modelling and Digital Survey) and Prof. Mariateresa Galizia (co-supervisor,
Professor of Architectural Drawing). Regarding the task related to structural
analysis, the work was co-supervised by Prof. Ivo Caliò (Professor of Dynami-
cs of Structure at UniCT) and Engineer Marco Intelisano (Structural Engineer
and Computational Design expert).
The development of the methodologies and workows applied in the case
studies shown in this thesis were obtained through visiting and collaboration
with other European research institutes:
• Ph.D. Visiting Student (2020) as part of an ERASMUS+ Traineeship
project at the Virtual Environments Lab Science and Technology in Ar-
chaeology Research Center (STARC), of the Cyprus Institute (Nicosia,
Cyprus)
• Ph.D. Visiting Student (2022, remotely) at the Technical University of
Delft (TU Delft), 3D geoinformation group
• Ph.D. Visiting Student (2022) at the 3D Optical Metrology unit at the
Bruno Kessler Foundation (FBK)
In April 2021, an application developed within the Ph.D. thesis was awarded
the Best Paper Award - 3D Modeling & BIM | Digital Twin 2021.
In March 2022, an application developed within the Ph.D. thesis was awarded
the Best Paper Award - 3D-ARCH 2022.
The case studies where the methodology proposed has been tested include
a minor urban center, Fleri (Catania, Sicily), which was hit by an earthquake
in Christmas 2018, and two blocks of the historic urban center of the city of
Catania (Sicily).
01.INTRODUCTION
34
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
1.7 Thesis Structure
The thesis is structured as follows:
Part I - Introduction and Background
Chapter 1 introduces the topic of the thesis, denes the research gap and
thus the aim and objectives of the research.
Chapter 2 deals with a summary of the evolution of urban surveying in Italy,
the evolution of parametric modeling, the methodologies to date adopted
for seismic risk analysis at the urban level, and the state of the art in City
Information Modeling.
Part II - Methodology proposal
Chapter 3 introduces the methodology proposed in this thesis by con-
textualizing it with respect to the current academic debate. In addition, it
introduces the reference standards and main workows.
Part III - Case studies, Results and Discussion
Chapter 4 discusses case studies involving the denition of City Informa-
tion Models at both spatial and urban scales. A comparative analysis is
carried out between dierent techniques of digital expeditious acquisition
for urban canyons and the Scan-to-FEM procedure is exposed, which al-
lows mesh models to be obtained from parametric models to be used for
structural analysis.
Chapter 5 presents the advancements from the state of the art through a
discussion of the results.
Chapter 6 closes the thesis with conclusions and future developments of
the research presented.
35
01.INTRODUCTION
36
REFERENCES
Baculo Giusti A. (eds.). (1994). Napoli all’innito/Naples in progress Napoli in assonometria … e Napoli nel data
base Una veduta della città ed una catalogazione informatizzata del patrimonio ambientale e architettonico.
Naples: Electa
Batty, M., (2018): Digital twins. Environment and Planning B: Urban Analytics and City Science, 45(5), 817–820.
DOI: https://doi.org/10.1177/2399808318796416
Boido C., Davico P. & Spallone R. (2021). Digital Tools Aimed to Represent Urban Survey. In (a cura di) Mehdi
Khosrow-Pour, Encyclopedia of Information Science and Technology, n. 5, pp. 1181–1195. IGI Global, Hershey,
ISBN: 9781799834793. <http://dx.doi.org/10.4018/978-1-7998-3479-3.ch082>.
Calvano, M., Empler, T. & Caldarone, A., (2019): L'ARIM per la prevenzione del rischio sismico. Disegnare Idee
Immagini, 59, 70-81
Caniggia G., Maei G. L. (2008). Lettura dell'edilizia di base. Florence: Alinea.
Coppo D., Boido C. (2010). Rilievo Urbano, conoscenza e rappresentazione della città consolidata. Florence:
Alinea
Funari, M., F., Hajjat, A., E., Masciotta, M., G., Oliveira, D., V., 2021. A Parametric Scan-to-FEM Framework for the
Digital Twin Generation of Historic Masonry Structures. Sustainability 2021, 13, 11088. https://doi.org/10.3390/
su131911088
Garzino G., Novello G., Bocconcino M. M. (2019). Handbook of Research on Urban and Territorial Systems
and the Intangible Dimension: Survey and Representation. In Carlo Inglese C., Ippolito A. (eds). Conservation,
Restoration, and Analysis of Architectural and Archaeological Heritage, pp 346-385. IGI Global, Hershey, ISBN:
9781799834793 DOI: 10.4018/978-1-5225-7555-9.ch014
Geiß, C., 2015. Seismic vulnerability assessment of built environments with remote sensing. PhD Dissertation,
Humboldt-Universität zu Berlin, http://dx.doi.org/10.18452/17104
Greco, A., Lombardo, G., Pantò, B. & Famà, A., 2020. Seismic Vulnerability of Historical Masonry Ag-
gregate Buildings in Oriental Sicily, International Journal of Architectural Heritage, 14:4, 517-540, DOI:
10.1080/15583058.2018.1553075
Grilli, E., Poux, F., Remondino, F., 2021. UNSUPERVISED OBJECT-BASED CLUSTERING IN SUPPORT OF SU-
PERVISED POINT-BASED 3D POINT CLOUD CLASSIFICATION. The International Archives of the Photogram-
metry, Remote Sensing and Spatial Information Sciences. XLIII-B2-2021. 471-478. 10.5194/isprs-archives-XLII-
I-B2-2021-471-2021
Ledoux, H., K. A. Ohori, K. Kumar, B. Dukai, A. Labetski, and S. Vitalis (2019). CityJSON: a compact and easy-to-
use encoding of the CityGML data model. Open Geospatial Data, Software and Standards 4.4.
Milano, G., (2013). Quaderni di protezione civile: Cos’è il terremoto? Regione Molise, Osservatorio Vesuviano –
I.N.G.V., Napoli, pp. 1-28
Ministry of Culture, 2021. Link: https://www.soprintendenzapdve.beniculturali.it/la-soprintendenza-informa/at-
ti-di-indirizzo/linee-guida-per-la-valutazione-e-riduzione-del-rischio-sismico-del-patrimonio-culturale/ Last ac-
cess 7th December 2021
OGC CityGML 3.0, Conceptual Model, link: https://github.com/opengeospatial/CityGML-3.0CM Last access
15th June 2022.
37
Regione Piemonte, 2018. Link: www.regione.piemonte.it/web/temi/protezione-civile-difesa-suolo-opere-pubbli-
che/calamita-naturali/emergenze-sismiche-censimento-danni/sistema-erikus. Last access 7th December 2021
Sulzer, R., Nourian, P., Palmieri, M., Gemert, J., 2018. Shape based classication of seismic building structural
types. ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences.
XLII-4/W10. 179-186. 10.5194/isprs-archives-XLII-4-W10-179-2018
Vernizzi C. (2006). Parma e la via Emilia. Città storica, città moderna e asse fondativo: rilievo e rappresentazione.
Fidenza: Mattioli 1885
38
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
39
02.
40
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
BACKGROUND
41
The research questions and objectives outlined in the introduction set a high
degree of interdisciplinary nature of the work itself. The topics covered from
geomatics to parametric modeling and 3D city models with applications con-
nected to expeditious seismic assessment. In addition, the specicity of the
sites chosen as prototypes of the proposed methodologies add characteristi-
cs and conditions that determine advancement in the state of the art and thus
make hard an exact overlap with previous case studies both in complexity
and in the topics covered. Therefore, works in the literature were examined
by prioritizing the methodological prole and resources required. This is due
to the fact that the topics covered intersect multiple elds and many expe-
riences despite being applied for purposes that are dierent from the seismic
analysis are still relevant to the objectives of the thesis.
The state of the art will be discussed in this chapter by dividing it according to
the subjects covered. Each section presents an oversight of the main concep-
ts with respect to the topic discussed going on to specify in more detail the
aspects of interest that dene gaps in the research and compose the back-
ground of the work presented here. The purpose of this chapter is to provide
an overview on the topics of urban digital surveying, methodologies related
to parametric modeling, and the evolution of seismic analysis in southern
Italian territories. Having dened these fundamental themes, the main topic
of 3D City Models and City Information Modeling is introduced, which is su-
bordinate to the themes discussed above and constitutes the main reference
framework for the work presented in this thesis. The order of exposition of the
topics covered in the state-of-the-art review mirrors the order of processes
related to reality-based parametric information modeling: acquisition, para-
metric modeling, information enrichment and analysis.
Therefore, this chapter will discuss the main concepts related to digital sur-
veying with an in-depth discussion related to urban surveying (with reference
to Italian schools of thought) and articial intelligence-based segmentation
and classication techniques (section 2.1). This is followed by an introduction
to parametric modeling with insights related to its evolution (implicit and expli-
cit modeling), the evolution of Visual Programming Languages (VPL), and the
adoption of Grasshopper as a digital environment for urban modeling (section
2.2). An overview is then presented on the extant schools of thought related
to seismic risk analysis on an urban scale with a focus on studies carried out
in Catania, Italy, and current implementations of seismic analysis processes
such as Scan-to-BIM-to-FEM and parametric applications with VPL (section
2.3). In conclusion, the topic of 3D City Models is introduced with reference
to denitions, standards, and the methodology of City Information Modeling
(section 2.4).
02.BACKGROUND
42
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
2.1 Digital Urban Survey: evolution and
techniques
The city is a complex evolutive system that adapts to the history of places
and also is interpreted and modied by successive generations. In the evo-
lution of historicized urban centers, there are many repeated constants and
many variables, consisting mostly of new readings of places, new interpreta-
tive approaches that represent the change that allows the system to adapt.
In this context, the role of urban survey, as highlighted by Dino Coppo, is to
identify, highlight, recognize, relate, describe and represent, the set of values
present in the investigated area with the aim to build the formal and structural
invariants present in the construction of the image of a city (Coppo & Boido,
2010). In this perspective, urban survey can be envisioned as a multi-layered
knowledge system, open and implementable over time.
The principal distinctions between urban surveying and architectural sur-
veying concerns the evolutionary nature itself of any urban area. Indeed, the
pattern of a consolidated urban area or a historical town center has evol-
ved to its present state of denition as a consequence of a number of in-
terventions, whether they be for the purposes of completion or restoration,
which are dicult (if impossible at all) to associate to documentable design
hypotheses. Even though a shortcoming of data can also be encountered in
architectural surveying, it cannot be compared to urban surveying. The main
dierence therefore lies in the more pronounced character of surveying as a
continuously evolving process which therefore rarely nishes with the end of
a project. Therefore, the idea of surveying within an urban context of multi-
layered forms and structures must be understood as an open investigative
process. This goal can be pursued by adopting a holistic and interdisciplinary
approach including surveying, history, urban planning, architecture, sociolo-
gy, and administrative policy methods. The overall quality of an urban survey
is not related to the number of information collected but to the level of deve-
lopment of an open survey methodology that makes use of various surveying,
cataloging, and data processing methods and is accomplished by breaking
down the complexity under study into particular subsectors (mostly dened
by the scope of the survey) while guaranteeing exibility and responsiveness
for critical reconstruction or interpretation of the data gaps object under eva-
luation (Coppo, Boido, 2010).
According to the traditional methodologies referenced by Italian schools of
thought, the analytical process of the urban survey can be broken down into
the following basic stages: identication of the cartographical base, classi-
cation and representation of morphological and formal construction cha-
racteristics, creation of technical maps with temporal sequences of individual
construction events and related socio-political motivations, the graphical de-
nition of the overall formal image, and identication of the goals behind the
proposed intervention with the creation of the relative database.
It is worth noticing that the denition of a proper database that stores all
the data gathered until that moment is the last phase of this methodology
43
together with the denition of the scope of the survey itself (the intervention).
This approach depends heavily on the analogical tools used for the represen-
tation and management of the collected data. The diculty in modifying the
represented objects pushed towards a basic workow for all scenarios with
an in-depth study of the database and the purpose of the survey only at the
end, so as to limit time-consuming and onerous modications. However, with
the innovation of CAD systems in the 80s, there was a progressive change
in the perception of this methodology. The new tools, which were digital and
fast, made it possible to drastically reduce the time for processing changes.
The rst change in the direction of urban 3D information systems came about
thanks to the eorts of the Turin drawing school headed by Augusto Cavalieri
Murat in 1974. The work in question concerned the study of the historic cen-
ter of Turin for the new urban plan. As a result of this pioneering work, it was
dened the standard described by UNI 7310 that would set the course for
database-driven three-dimensional urban modeling. For the survey of Turin,
the paper planimetry of the Gatti cadastre on a scale of 1:1000 was used as a
working basis. On this were reported the results of visual surveys conducted
on the elements listed above (including, for example, the number of oors,
the typology of the buildings’ summit cornices and basements, and the po-
sition of windows) using the symbols that would become those of the UNI
standard. This representation better featured 3D information in a 2D paper
medium, which changed the perception of the building in the study phase.
However, the paper support of a 1:1000 scale representation only allowed
containing a certain number of symbols, thus limiting the description of the
city’s complexity (Coppo, Boido, 2010).
Therefore, this has meant that until the 1990s, as will be seen, analyses of
urban areas gave centrality to the cartographic support (which was enriched
with meanings through the synthetic capacities of symbols or legends), star-
ting with the Neapolitan experience conducted by Adriana Baculo Giusti it
is possible to to see the separation of content and drawing. In this new per-
spective, graphic representation begins to become a representation of rea-
lity to which specic qualities borrowed from a database separate from the
drawing itself can be conferred. The work consists of representing the city of
Naples in axonometry. It was realized over a period of 20 months by a wor-
king group led by Baculo and comprising 80 architects. In order to make the
work homogeneous, it was necessary to create an abacus of signs that deter-
mine the technical and formal characteristics of each building. The presence
of these signs allowed the drawing to be deconstructed semantically within
a GIS database that enabled each individual building to be easily understood
(Baculo Giusti, 1994) (g. 1).
02.BACKGROUND
44
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 1 | Left: a table of the
axonometry of Naples. Right:
detail of the axonometry on
Piazza Plebiscito. Source:
Baculo Giusti, 2000.
From the end of the 1990s onwards, satellite technologies and GIS systems
combined with CAD environments enabled the development of software de-
dicated to the management of representations on a territorial and urban sca-
le. From a methodological point of view, there is a reversal in the order of the
phases. In fact, as Coppo warns, the risk appears to be that of postponing
the moments of analysis and verication of hypotheses until the end of the
entire survey and restitution process, with the risk of losing sight of the real
objectives of the survey operations themselves. Before the advent of digital
culture, the practice of manual drawing obliged to develop these hypotheses
and transform them into theses during surveying and restitution operations.
Today, on the other hand, there is too often a tendency to accumulate a quan-
tity of data that is often redundant and sometimes insucient for the intended
purposes. (Coppo, 2010) (g. 2).
Fig. 2 | Conjectural philological
survey of Piazza Savoia in Turin.
Source: D. Coppo, C. Boido,
2010.
45
The surveying of urban centers has always represented, in the history of
surveying techniques one of the most important applications. Even today,
the production of cartographic instruments still constitutes the majority of
applications of surveying techniques. The cartographies of the Roman era,
drawn up for cadastral purposes, undoubtedly represent a paradigm that is
still widely applied today in the production of technical cartography on an
urban scale. After the Middle Ages' iconographic phase, cities returned to
being represented metrically in a scientic manner during the Renaissance.
The creation of new cadastres at the end of the 17th century (such as the
Napoleonic cadastre) initiated the evolution of increasingly advanced land
mapping techniques that led to the development of more precise and reliable
instruments. The second half of the 19th century witnessed the development
of photogrammetry (thanks to the studies of the French Colonel Aimé Laus-
sedat at the Academy of Sciences in Paris), which would be further conso-
lidated as the main tool for land mapping thanks to its use for military pur-
poses during the First World War. In Italy, photogrammetry began to be used
methodically (for civil purposes) in 1938 for the production of the rst modern
urban mapping maps.
It is possible to assume the rise of the digital survey for urban representation
purposes with the use of digital images instead of analog images in photo-
grammetry since the early 1980s. Although digital images existed before, it
was not until the early 1980s that their technology reached a quality that was
comparable to the results obtained with analog images. The digital progress
of the 1980s facilitated the ability to develop reliable and self-contained al-
gorithms for the correlation of digital pictures, which in turn made it possible
to automate the dicult tasks that had to be completed before the real tho-
rough survey could begin, such as the external orientation of the images, or
determining the geometric characteristics required for a precise estimation of
the coordinates of the places of interest. In addition, these digital procedures
reduced the cost of expertise needed since the software could handle many
tasks once dealt with by several surveyors (Rinaudo, 2010).
Equally important was the evolution of LiDAR (Light Detection And Ranging)
and CMM (Coordinate Measuring Machine) technologies designed during the
1950s mainly for military and aerospace purposes. It was not until the 1960s
that these technologies were made available for civil use, but only for ba-
sic measurement purposes. The evolution of these systems continued in the
projects of NASA, which exploited this technology to begin mapping opera-
tions on the lunar surface under the Apollo 15 project. Only with the full ma-
turity of GPS systems these technologies see massive will be used in the civil
sector. Although the development of these technologies predates that of di-
gital photogrammetry, compared to the versatility of the latter, laser scanning
will become a widespread and eectively used technology from the 1990s
(the rst commercial airborne LiDAR system was developed in 1995) onwards,
before booming in the early 2000s (Petit, 2020; Wang et al 2020) (g. 3).
02.BACKGROUND
46
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 3 | Every first-generation
scanner that was marketed
from 1998 to 2000 was control-
led via connection to a laptop.
Riegl’s original LMS-Z210 is
shown with Dr. Andreas Ullrich
(left) and Dr. Johannes Riegl
(right). Source: https://www.
xyht.com/lidarimaging/early-3d-
scanning-competition-1998-
2000-part-13/
Following on, the author describes technologies and approaches that, with
respect to the objectives of the thesis, were considered to be of reference
for the digital survey operations conducted during the thesis work (section
2.1.1). The chapter concludes with an in-depth examination of the techniques
of segmentation and semantic classication with articial intelligence (section
2.1.2) of point clouds obtained through digital surveying, which provides the
research context for the application described in section 4.2.2.
2.1.1 An overview of digital surveying
technologies and approaches
Nowadays, digital tools give precious support to the investigation on complex
urban contexts and processes, generally characterized by articulate relation-
ships between multiple aspects (Boido et al, 2021; Galizia, Santagati, 2012).
Moreover, the fragility of historical city centers requires specic methodolo-
gies that respond to the need to have a rapid mapping of the investigated site
(Predari et al, 2019) and/or models for the simulation or the management of
critical scenarios (Bocconcino et al, 2021; La Russa, Santagati 2020).
The opportunities related to digital surveying are numerous. Thanks to the
new tools of metric data acquisition, there is a considerable reduction in error
(at the scale of mm) and also its quantication ascertained by the manu-
facturers. The in-situ survey phase becomes faster, quicker, and with greater
quantity and quality of the measured data. The digital approach also allows
the simultaneous acquisition of data belonging to dierent themes. An exam-
ple is the survey of geometric, calorimetric, and reectance data simultane-
47
ously using a single sensor. The georeferencing to scale, the objective quality
of the data, and the possibilities of sharing, management, and verication are
just some of the other features that make the new digital approach ecient
and eective at the same time.
The product of this survey is, most of the time, a three-dimensional model
(mainly composed of points) which provides excellent reproduction of the real
artifact. The approach to the survey phase becomes more objective on the
part of the operator since the interpretative-subjective phase, typical of the
traditional approach, takes place at a later stage. This prevents the operator
from making semantic choices in the restitution of the artifact (Migliari, 2001).
The three-dimensional model mentioned above is also called a ‘point cloud’.
Currently, such a model can be produced from dierent types of instruments
that can be divided into passive optical sensors and active optical sensors
(Caroti et Al., 2015).
Passive optical sensors make it possible to acquire the geometry of the
object by observation only. These optical instruments capture the reected
light from the object (i.e. its image) and reproduce it in a 2D image (hence are
dened as image-based). Among the dierent acquisition methodologies re-
lated to passive optical sensors, there is a particular one that allows to obtain
point clouds: multi-image photogrammetry. Through the capture of several
images with sucient overlap between consecutive shots, it is possible to
obtain three-dimensional information. Nowadays there are many software
applications that: apply algorithms for the automatic recognition of homo-
logous points in dierent photos; automate internal and relative orientation
procedures (necessary for the use of photogrammetric techniques); produce
three-dimensional models made up of points (with RGB information) and from
there they return ‘image products’ (such as orthophotos or mosaics) or ‘vec-
tor products’ (cartographies, proles, etc.) (Remondino, Rizzi, 2010) (g. 4).
Fig. 4 | Schematic drawing of
photogrammetry. Source: Li et
al., 2013
02.BACKGROUND
48
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Active optical sensors, on the other hand, measure a distance from the instru-
ment to the point to be detected. Such instruments use the electromagnetic
radiation of the laser to measure, via a light emitter and a light receiver, the di-
stance mentioned above. The product is a point cloud that in addition to RGB
information has other attributes (such as reectance) depending on the de-
vices mounted on the scanner (e.g. thermal imaging camera: thermographic
data). These high-performance scanners allow a considerable reduction in
error (a few mm even at a distance of tens of meters) and it is possible to
know in advance (from reading the technical specications) the error commit-
ted by the instrument. In this way, it is possible to design the shots in advance
in such a way as to guarantee the necessary accuracy throughout the acqui-
red model. Similar to passive optical sensors (although for dierent reasons),
laser scanners nd it dicult to acquire in the presence of reective surfaces
(such as mirrors, windows, and certain types of metal surfaces) (Vosselman
et al., 2003) (g. 5).
Fig. 5 | Laser scanner operating
principles: triangulation (left)
and range scanners (right).
Source: Guidi, Remondino,
2012.
A digital survey project that can be dened completely in all its parts can fore-
see an initial phase where the territorial reference systems are dened (useful
for georeferencing purposes), then data acquisition is carried out by choo-
sing the most suitable survey techniques with reference to the context and
the nature of the object (laser scanner, photogrammetry, direct survey, aerial
survey). Once the survey campaign is over and all the data are stored in the
most appropriate devices (usually dealing with huge masses of data in the or-
der of Gb), the ltering and processing phase begins, which aims to eliminate
redundancies and data noise (typical of complete survey systems). Once the
survey datasets have been cleaned up, the registration of photographs and/
or scans can be undertaken. This last step delivers a digital product in the
form of a point cloud that could be further improved by semi-automatic and
manual operations of cleaning (deleting unreliable and noisy portions of data).
Finally, the cleaned point cloud is ready to be used as a digital hub from which
to export dierent technical representations according to the error obtained
during the registration phase (La Russa, 2019).
Next, there is a summary of techniques and case studies related to both
passive and active sensor-based procedures and technologies adopted for
dealing with urban-scale objects. In particular, the case studies reported have
49
been selected specically for their remote sensing approach since they pro-
ved to be relevant to the research goals addressed in this research project.
The last years have seen a growing development of technologies and sen-
sors for fast 3D acquisitions which allow expeditious mapping campaigns
in very complex environments. The fastest solution for mapping the city can
be envisaged in vehicle-mounted MMS, although they are not very suitable
in presence of high-density historical urban centers. Indeed, they are cha-
racterized by pedestrian and narrow streets, buildings of dierent heights and
varied street ratios (Barrera-Vera, Benavioles-Lopez, 2018). In these cases, it
is preferable to adopt other solutions such as spherical photogrammetry or
iMMS tools. The latter is based on SLAM algorithms that allow capturing 3D
point clouds in real-time by walking in the area of interest. The drawback is
related to the low levels of density and accuracy of these sensors (compared
to TLS solutions), as well as the drift errors along the trajectory that may af-
fect the global accuracy. These new devices are constantly evolving techno-
logies that need to be assessed against more consolidated techniques such
as photogrammetry and TLS. In literature can be found dierent approaches
to evaluate iMMS data both in indoor and in outdoor contexts (Nocerino et al,
2017; Sammartano, Spanò, 2018; Chiabrando et al, 2018; Sammartano et al,
2021; Salgues et al, 2020; Marotta et Al., 2022) (g. 6).
Fig. 6 | ZEB Horizon LiDAR
SLAM-based scanner with
backpack mobility option for
indoor and outdoor mobile
mapping. Source: https://
www.aniwaa.com/buyers-gui-
de/3d-scanners/slam-3d-scan-
ners-imms-mobile-mapping/
As regards spherical photogrammetry, in their studies Abate et al (2017) and
Barazzetti et al (2018) demonstrate that accurate metric reconstructions can
be achieved using low-cost sensors. In addition, Teppati Losè et al (2021)
verify in their work the integration between iMMS and spherical photogram-
metry for the survey of the Montanaro bell tower. Photogrammetry 360 is
also and especially a valuable tool for urban expeditious surveying due to the
possibility of capturing frames through video acquisition. The high resolution
that 360 cameras are achieving allows for excellent density overlays of auto-
matically obtained point clouds (Barazzetti et Al., 2022) (g. 7).
02.BACKGROUND
50
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 7 | Example of a 360 ca-
mera, the Xiaomi Mijia Mi Sphe-
re 360 (top-left); reconstruction
of a planar wall with spherical
camera (bottom-left); 360 SFM
survey of a spire (right). Source:
Barazzetti et al., 2018.
Since the Blk2go is new in the market, few studies about its performance
are currently available. In particular, one is addressed at the evaluation of its
usability for the generation of DTM in highly vegetated areas for detecting and
documenting archaeological anomalies (Limongiello et al, 2020); another one
concerns narrow spaces such as indoor corridors or multi-store buildings
(Piniotis et al, 2020). In both cases, the outcomes of the evaluation return an
accuracy that represents scales of 1:100 or 1:200, fully compatible with the
aim of this research work (g. 8).
Fig. 8 | BLK2GO Handheld
Imaging Laser Scanner. Source:
https://leica-geosystems.com/
en-gb/ products/laser-scan-
ners/autonomous-reality-captu-
re/leica-blk2go-handheld-ima-
ging-laser-scanner
In addition to these references, which served to determine the context of the
research with regard to the remote sensing issues covered, there are other
case studies in section 2.4.3 which, were preferred to be placed in the di-
scussion of 3D City Models as they are less focused on remote sensing but
equally of interest for the acquisition techniques used in the urban environ-
ment.
51
2.1.2 Articial Intelligence for Cloud
Segmentation and Classication
As seen in the previous section, the adoption of digital acquisition techni-
ques is now an established practice in both industry and research. Currently,
acquisition-registration workows are increasingly automated, speeding up
work and reducing errors. However, the use of the 3D models obtained has
remained unchanged for a long time as the point clouds obtained are used as
static references for generating views and sections, thus reducing the poten-
tial of the digital product itself.
For this reason, 3D data categorization has recently been a very active stu-
dy area as a result of the 3D models' steadily expanding use in a variety of
applications. It has gotten increasingly important in a variety of applications
and domains, including robotics (Maturana et al., 2015), autonomous driving
(Wang et al., 2017), urban planning (Xu et al., 2014), heritage (Grilli and Re-
mondino, 2019), geospatial (Özdemir and Remondino, 2018), etc., to auto-
matically group huge data into many homogenous regions with comparable
qualities. The objective is to automatically classify semantically continuous
portions of point clouds (e.g. walls, windows, columns, etc.) in order to op-
timize modeling operations on point clouds with automatic and semi-auto-
matic workows. The means by which this can be achieved is through the
use of Articial Intelligence in geomatics, in particular by adopting Machine
Learning (ML) and Deep Learning (DL) techniques (Grilli et Al., 2019; Matrone
et Al., 2020) (g. 9).
Fig. 9 | 3D classification pro-
cess based on artificial intelli-
gence: surveyed point cloud (a),
automated features extraction
(b), manual annotation of a
small portion to define classes
(c), final automated classifica-
tion results (d). Source: Grilli et
al., 2019.
ML is a branch of statistical learning. It works through the use of classiers
such us Support Vector Machine (SVM) and Random Forest (RF). These clas-
siers are trained using a collection of features and training data with asso-
ciated label information (i.e. classes). An attribute that is useful or signicant
to the classication process is dened as a feature, which, in the case of point
clouds, might be geometric or radiometric. The denition of the right features
is fundamental to obtain a training phase ecient enough to semantically
segment the full dataset based on the prediction of the classier used. Ex-
tracting and/or generating the right features can sensibly change the results
obtained.
02.BACKGROUND
52
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
3D features in the case of 3D point cloud data generally derive from a parti-
cular geometric property of the global or local distribution of the points, and a
substantial number of them have been proposed in the literature (Georgianos
et al., 2015; Guo et al., 2016; Weinmann et al., 2014). The covariance matrix
of the 3D point coordinates in a certain neighborhood of points is used to
produce the most prevalent 3D features utilized to characterize the local ge-
ometric behavior of the point cloud.
Following is a description of the methodology used as a reference in this
research project and which constitutes the framework adopted in one of the
applications described in Chapter 4. In particular, the methodology adop-
ted is based on Random Forest as described in Grilli et Al., (2019). It can
be summarized in the following steps: (i) neighborhood selection, (ii) featu-
res extraction, (iii) features selection, (iv) manual annotation, and (v) classi-
cation (Weinmann et al., 2016). Initially, distinct geometric characteristics
are extracted at various scales. Then iteratively evaluate just the more perti-
nent characteristics and re-run the classication procedure after conducting
a multi-scale classication with a Random Forest classier. Last, using the
standard confusion matrix ratings, the various ndings are compared (g. 10).
Fig. 10 | Classification workflow
(left) and its extension (right)
to evaluate features relevance
for the classification process.
Source: Grilli et al., 2019.
The features extracted for the training of the classiers are the ones of the co-
variance matrix related to local regions of the point cloud to classify (Chehata
et al., 2009). The values of these features emphasize certain topological and
geometrical characteristics of the cloud depending on whether the individual
regions of the point cloud are analyzed by giving priority to linearity, planarity,
or volumetry of the neighborhood under consideration. In the gure below the
complete list of the features taken into account (g. 11).
Fig. 11 | Considered local 3D
shape features/covariance fea-
tures. Source: Grilli et al., 2019.
53
The features considered can be extracted several times with dierent radius
size as concerns the 3D neighborhood (Niemeyer et al., 2014). This may in-
crease the possibility of success in the prediction as stated in Weinmann et
al. (2013) (g. 12).
Fig. 12 | Visual comparison of
some geometric features ex-
tracted ad hoc on the different
case studies. Source: Grilli et
al., 2019.
Regarding the classication phase, as mentioned above, it is carried out than-
ks to the Random Forest classier. RF is a supervised classication method
created by Leo Breiman (2001) that combines a group of classication trees,
gets a prediction from each tree, then votes on the top candidate. To create
the forest trees (as are called the processes of this method), two parame-
ters must be set: the number of decision trees to be dened (Ntree), and the
quantity of variables to be chosen and tested to determine the optimal split
during tree growth (Mtry) (Belgiu et al., 2016). The best F1-score calculated on
02.BACKGROUND
54
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
the test set is taken into account when tuning the Ntree and Mtry with reference
to an already labeled dataset (training phase). The advantages of RF consist
in being a very successful and robust method due to the presence of many
decision trees. Furthermore, problems such as overtting (typical in ML) are
mitigated since the nal prediction is an average of the predictions of the
individual trees. RF also makes it possible to assess the actual eciency of
each feature used, thus being able to see how much a particular feature was
decisive for the prediction. This provides an enormous advantage in the fea-
ture extraction/engineering phase (Grilli et Al., 2019).
The procedure ends with the evaluation phase. A small portion of the dataset
under examination is selected to be manually classied. Subsequently, the
classier is applied to this portion, which is called the test set. The evalua-
tion consists of comparing, within a confusion matrix, how many instances
have been correctly classied. The rows of the matrix represent the predicted
instances while the columns represent the actual instances. For each class,
there are indicators that help determine the eciency of the classication,
in particular there are the precision (indicating the quality in identifying the
class), recall (the completeness of the classication) and F1 score, which
consists of an average of these two and provides an all-round indicator. In the
context of geomatics, this indicator (F1 score) is signicant for the evaluation
of the nal result (g. 13, 14, 15).
Fig. 13 | Definitions of Preci-
sion, Recall and F1 Score. Tp
= true positive, Tn = true nega-
tive, :Fp = false positive, Fn =
false negative. Source: Grilli et
al., 2019.
Fig. 14 | A portion of point
cloud manually labelled with
8 classes. Source: Grilli et al.,
2019.
55
Fig. 15 | Example of a confu-
sion matrix obtained from a
case study using 8 geometric
ad hoc features plus the height
information (Z coordinate).
Source: Grilli et al., 2019.
A meaningful case study for the objectives set in this research project is the
classication of some porticos in the historic center of Bologna where the
described methodology was applied (Remondino et Al., 2016; Grilli et Al.,
2019). About 1.2 million points constitute the Bologna dataset. This point
cloud incorporates a variety of geometric forms, varied materials, and nu-
merous architectural details including mouldings and decorations. Thirteen
distinct classes are selected and annotated for the categorization goal (g.
16). In order to overcome the diculty of the classication task, RGB va-
lues were additionally considered as essential components for the successful
classication of the point cloud in addition to the 11 geometric characteristics
extracted. The F1 score achieved is 79.82 %.
Fig. 16 | Porticoes point cloud
(a), annotated (b) and classified
(c). Source: Grilli et al., 2019.
02.BACKGROUND
56
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
The relevance of this case study with reference to the work presented in this
research project lies in the fact that the point cloud is developed from terre-
strial photogrammetry whereas in most cases this methodology is applied to
airborne lidar scans. In the case study, on the other hand, the architectural
features of an urban canyon in an Italian historic centre were highlighted.
However, it is worth pointing out the limitations of the research. This metho-
dology includes the possibility of taking advantage of the training done on a
case study, on other objects (the so-called transfer learning). Unfortunately,
the limitation lies in the uniqueness of the historical architectural heritage of
cities. In fact, the covariance features and RGB values depend on the geome-
tric and material characteristics (i.e. the architectural style) of the case study
considered. For example, the same training could not be used with the same
classes and the same training in the urban canyon of a modern city. There-
fore, at present, this methodology, both in the architectural and urban case,
must be repeated from the very beginning. On the other hand, a strong plus
is that once an urban area with similar characteristics has been identied (as
is the case in most historic city centers), the training developed can be reused
and implemented in order to obtain better and better results in a shorter time.
57
2.2 Parametric Modelling
In reality capture based workows, digital surveying is often the starting point
and not the nish line. Indeed, the digital products obtained are often used
as references for the creation of digital information models that are metrically
accurate to the levels of detail indicated. This activity belongs comprehensi-
vely within the parametric modelling range of techniques where it is possible
to generate models, generally three-dimensional, which present a semantic
structure between the parts at both geometric and informative levels.
In this section, the main concepts related to parametric modelling are pre-
sented, dening its purposes and main categories (implicit and explicit), and
then goes on to explore the use of Visual Programming Languages as a digital
environment suitable for geometric, informative and exible management for
the creation and manipulation of urban models.
Briey, the term 'parametric modelling' refers to the relationships existing
between all the elements of a model, which allow coordination and change
management operations to be performed. Relationships can be created au-
tomatically by the software or by the user. In mathematics and mechanical
design CAD systems, the numbers or characteristics that dene this type of
relationship are called parameters. Changes made at one point are exten-
ded to the entire model (Autodesk, 2022). However, it is an oversimplication
to dene parametric modelling as the result of digital technologies aimed
at speeding up representation processes. The very concept of 'model' and
'parametric' encompass a true theoretical revolution that has its roots in the
history of architecture and representation. Migliari, on the occasion of the
rst digital survey of the Colosseum, revised the very concept of drawing and
model:
“Drawing as Model, or rather, Model, idea, which is generated in the changing
forms of drawing” (Migliari, 2004).
Methodologically, drawing became more a procedure than a product, as an
idea never completely dened. In this way, the result of drawing is a exible
product (thanks to digital instruments) that is constantly evolving, similar to
what has already been theorised in the eld of surveying disciplines (as de-
scribed in section 2.1).
Today, thanks also to the contamination with other disciplines, there are new
model denitions that clarify what Migliari already dened. In a lecture in
2017, Mateusz Zwierzycki dened a model as: “a system that describes a
part of reality, dening its internal relations and properties” (Zwierzycki, 2017).
The adjective ‘parametric’ derives instead from an architectural movement
that saw its beginnings in the 1940s with the denition of parametric archi-
tecture. One of its rst denitions was by architect Luigi Moretti, according
to whom parametric architecture is dened by: “[...] parameters and their re-
02.BACKGROUND
58
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
lationships [...] the code of the new language of architecture, the ‘structure’
in the original sense of the term [...]”(Bucci, Mulazzani, 2006). The architectu-
ral project is no longer seen as an overlapping of sub-projects (architecture,
structure, systems, etc.) but as an organic whole of functions and require-
ments that determine its form and function (g. 17).
Fig. 17 | Stadium model with
isoview study, Luigi Moretti.
This stadium assumes a shape
that allows equal visibility for
each spectator. Source: Bucci,
Mulazzani, 2006.
There is thus a move towards a more uid concept of architecture based
on two fundamental concepts: parameters and relationships. This concept,
more than half a century later, will dene the basic structure of all parametric
information models currently available in the AEC industry (g. 18). In conclu-
sion, a parametric model means a system that describes a part of reality, with
its attributes (both geometric and informative) and relationships, and that the
model is capable of changing its properties on the basis of xed or variable
parameters (Zwierzycki, 2017).
Fig. 18 | International Terminal
Waterloo - Nicholas Grimshaw
& Partners (London, 1994). One
of the first parametric model
realised combining first genera-
tion CAD environments and tex-
tual programming languages.
The model was able to change
its cross-section in relation with
the size of the railway platform.
Source: https://grimshaw.
global/projects/international-ter-
minal-waterloo/
59
2.2.1 Implicit and Explicit Parametric Modeling
The technical evolution of parametric modelling has gone hand in hand with
that of digitisation in architecture and construction. It is possible to identify
three phases, or levels, through which the evolution of parametric modelling
can be described (Calvano et Al., 2022). The rst level consists of the use
of industry-produced software that allows the user, by means of a simpli-
ed graphical interface and pre-established input rules, to construct linked
architectural components. This level corresponds to the beginnings of the
development of CAD systems (1950s) up to the modern Building Information
Modeling (BIM) systems that implement and facilitate the three-dimensional
management of both geometric and informational aspects of a building (Za-
bramski et Al., 2013; Eastman et Al, 2018). The potential of this system has
also extended to the existing cultural heritage by developing methodologies
such as Scan-to-BIM that allow manual and semi-automatic workows in
the transition from point cloud to a Historical-BIM model (HBIM) in which all
building components and documents are organised in a spatial 3D database
semantically dened. This methodology in particular enabled the digitasation
of complex activities related to historical buildings such as documentation,
conservation, management, design and maintenance (Osello et Al., 2018;
López et Al., 2018; Murphy et al., 2009) (g. 19, 20).
Fig. 19 | From scan to HBIM
to automated documentation.
Source: Murphy et al., 2009.
02.BACKGROUND
60
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 20 | Samples of H-BIM
object library. Source: Murphy
et al., 2009.
The second level concerns parametric modelling through the use of classical
Textual Programming Languages (TPL) such as C#, Python, and VBScript
etc. This level, although the least used, is the oldest of all as it is the core
through which software belonging to the rst level has been produced since
the beginnings of CAD. The limited use by architecture engineers of this level
is due to the fact that it requires a high level of programming knowledge that
professionals often do not possess because of their education. However, this
level was the only way of modelling complex surfaces in the 1980s and 1990s
and contributed greatly to the development of parametric architecture. Today
this level is still adopted for research and complex task in the industry (Calva-
no et Al., 2022; Zwierzycki, 2017; Caetano et Al., 2020) (g. 21).
Fig. 21 | Example of a complex
geometry modelled using only
Textual Programming Langua-
ge. Source: https://controlmad.
com/eng/formacion/cur-
so-python/.
The third level is a synthesis of the accessibility of the rst and the potential
of the second. This level, the most recent of the three, consists of the use of
Visual Programming Languages (VPL) which, through the use of graphic no-
des, allow even non-programmers to write code within parametric modelling
software. Despite the accessibility aorded by an entirely graphic interface,
61
the adoption of this level requires training that introduces some basic pro-
gramming concepts (such as object types and data structure). The deve-
lopment of this level, despite the fact that it can be traced back to the very
beginnings of the rst CAD prototypes, only became widespread from the
2000s onwards in an attempt to enable professionals to achieve results close
to those of the rst CAD prototypes. (Spallone et Al., 2019; Calvano et Al.,
2022) (g. 22).
Fig. 22 | Main window of
VPL Grasshopper with some
components (the fundamental
algorithms) and the canvas
(the place where components
are placed and connected with
input/output). Source: David
Rutten for Wikipedia.
Currently, the three levels just described are often combined, and depending
on the level of use of the same software, it is dicult to classify which para-
metric modelling paradigm is being used. For this reason, it is convenient to
divide parametric modelling into two macrocategories: implicit and explicit
(Calvano et Al., 2022).
The focus of implicit parametric modelling is the nal model in a digital en-
vironment. This model is meant to be consulted, updated and enriched over
time in accordance with the scope of the model. All parametric processes are
controlled by pre-set interfaces that have the ability to change the geometry
and information of the model objects through numerical and data restrictions
(Saggio, 2007; Turk, 2016). With the aim to situate the whole model in a wider
information context, parameters are also employed to enrich objects with
02.BACKGROUND
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
information attributes from other knowledge areas. The model is thought as a
common database where all professionals and domain experts can share and
store data. For this reason, information enrichment is greatly facilitated by the
ability to view model objects in both 3D and 2D views and to make changes
to their properties inside property windows that are activated by object se-
lection. In this scenario, the modeller is asked to take into account the level
of detail and information needed for each component so that he can delivers
the outcome requested (Eastman et Al., 2018). Parametric modelling of rst
level (CAD/BIM software) is mostly included in this category.
On the contrary, the focus of explicit modelling is on how the model is obtai-
ned (the procedure) and all possible obtainable models (the results) . Thanks
to explicit modelling, the user has the ability to dene the hierarchy between
the modelled elements by himself, thus fullling the design requirements of
the project closely. In addition, since the user develops a code and not pre-
dened building components, the dened procedure is always editable in
its structure and responsive to the input provided. Precisely for this reason,
explicit modelling is the most widely used in research and complex projects
in the industy (Calvano, 2019). The major dierence in comparison to implicit
parametric modelling is in that, by dening objects, relationships and para-
meters from the very beginning, the user is not subject to the commercial
choices dictated by software houses in the development of information pa-
rametric modelling software so he is free to decide which way is preferable
thus lightening the computational burden and complexity required to achieve
project goals. Parametric modelling of second (TPL) and third level (VPL) are
labelled in this category
For the reasons stated above, the paradigm of explicit parametric modelling
was chosen in this research, combining TPL and VPL when necessary. The
next section will introduce a brief overview of the evolution of VPLs and its
advantages in the AEC sector.
2.2.2 Visual Programming Languages:
evolution and advantages
In the 1980s, there was a great diusion of personal computers, but the ave-
rage user did not have programming knowledge and this limited the impact
of these technologies in dierent sectors. Programmers tried to improve the
user interface but not always the eorts in this direction were successful. This
condition led to research aimed at using graphics to facilitate programming
skills, leading to the birth of Visual Programming (VP) (Halbert, 1984). By eli-
minating syntax, the graphical method focused on workow, making visual
programming an ecient tool even for skilled programmers. The friendliness
of this method was also demonstrated by cognitive psychology, as the human
brain can process visual information using two hemispheres instead of one as
in other cognitive processes (Myers, 1986). In accordance with Brad Myers,
63
VPL can be dened as a “system that allows the user to specify a program in
a two (or more) dimensional fashion. Conventional textual languages are not
considered two-dimensional since the compiler or interpreter processes it as
a long, one-dimensional stream” (Myers, 1986). The rst VPLs for geometry
modeling purposes can be found In the late ’80s: Prismis (nowadays known
as Houdini) and ConMan (Haeberli, 1988) (g. 23).
Fig. 23 | Modelling a glass by
profile revolution in ConMan
(Connection Manager, 1988).
Source: Halbert, 1984.
In the 2000s there was a new success in parametric design with a subse-
quent spread of programming tools (ex. Grasshopper, Dynamo, Marionette)
for design purposes. The applications went far beyond that, as the new VPLs
allowed the management of entire workows (and data) even between die-
rent BIM enviroments thus enabling an high level of interoperability. VPLs for
architecture began to be recognized as programming languages capable of
facilitating operations that designers, engineers, and architects used to carry
out manually (Rutten, 2012). Together with the BIM revolution, these topics
started to be included in the training of young architects (Boeykens et al.,
2009) (g. 24).
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Compared to traditional programming, visual programming has a very favo-
rable learning curve in the short term. However, for more complex processes,
VPLs are limited because they cannot keep up with traditional programming
in the long term (Zwierzycki, 2017). Thanks to the community behind VPLs
such as Grasshopper, it is possible to use a series of plugins that increase the
potential of VPLs compared to their default setup. However, there is still a gap
in the long term, even if it is smaller than the previous one. An emblematic
case of this phenomenon is the introduction of Articial Intelligence applica-
tions in the eld of parametric modelling. Indeed, a variety of plugins have
been created that allow the transition to these new practices within VPLs by
reducing the knowledge required to apply them. These plugins enable the
user to use Machine Learning and Deep Learning tools, enabling increasingly
complex data processing practices. Applications range from design to opti-
mization in production processes. Although in some applications there is no
need for textual programming implementations, VPL shows limitations in the
long term.
2.2.3 Grasshopper for BIM, Geomatics,
Urban Modelling and FEA
The VPL Grasshopper (GH) was used for the parametric modelling applica-
tions in this research. This VPL comes with the modeling software Rhinoceros
which is a software widely used in the AEC sector by professionals but also
researchers. The reasons that led to this choice lie in the fact that GH is a
Fig. 24 | Gallery of some of the
most famous Visual Program-
ming Languages environments.
Source: author’s image.
65
VPL aimed very much at NURBS-type modelling, has a data structure that
lends itself to the semantic layering of elements, and is a widely used tool in
the academic community and thus permits comparisons with better scientic
rigor regarding the results (TUDelft TOI-Pedia, 2022).
Similar to all VPLs, Grasshopper has an interface consisting of blocks called
‘components’ which require both input and output for their operation. The
relationships between input/output objects and components are graphical-
ly achieved through the joining of wires that connect the objects involved.
This procedure reduces any kind of language syntax to a minimum, which
remains only for more advanced operations related to data structuring. The
foundation of grasshopper’s data structure is the ‘Data Tree’. This structure
is articulated analogously to a tree, so a Data Tree will have, advancing in
depth, ‘Branches’, ‘Lists’ and ‘Items’. As an analogy to TPL, one only has to
think of the nesting that occurs in JSON formats through the use of dictiona-
ries (TUDelft TOI-Pedia, 2022). The denition of a specic structure of a Data
Tree, dene the semantic relationships between the parts of the parametric
VPL-based model.
Another great strength of Grasshopper is the community of developers sup-
porting it. Through a dedicated portal called ‘Food4Rhino’, many plug-ins
developed for GH can be used. These plugins are mostly free of charge and
are often products of research. At the time of writing this thesis, the portal
has 1327 applications spread over various elds ranging from aerospace ap-
plications to medical applications and the various branches of engineering.
In relation to the topics of this thesis, it is interesting to highlight how many
apps have been developed around Grasshopper. There are 155 apps for BIM,
593 apps for Architecture, 91 apps for importing and exporting from GH, 24
apps for point clouds and reverse modelling, 80 apps for urban planning and
urban modelling and 105 apps for structural engineering (Food4Rhino, 2022).
Interoperability and freedom in both geometric and informational modelling
make Grasshopper an eective tool for the development of research related
to survey and urban modelling. To reinforce this hypothesis, there are also
several case studies in the literature, which are discussed further in section
2.4.3.
02.BACKGROUND
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
2.3 Urban Seismic Risk Assesment:
the Southern Italy scenario
Although the objectives of this thesis are not directly concerned with con-
ducting seismic safety analyses on a neighbourhood scale, but with procedu-
res for obtaining a structural geometric model, it is nevertheless useful to in-
vestigate what procedures are adopted by local institutions and researchers
at the urban scale for determining the level of seismic safety. In particular, in
this section, the author will provide information on the context of these proce-
dures in relation to the territory that hosts the sites of the case studied where
the proposed methodology has been tested. Furthermore, this overview can
highlight how data usually are acquired in order to speed up the analysis wor-
kow as stated in the Introduction.
First, the schools of thought that are adopted at a national level in the study of
urban seismic safety will be introduced (section 2.3.1). Secondly, some pre-
vious studies relating to the city of Catania, which is representative of typical
urban scenarios in southern Italy, are briey listed (section 2.3.2). After having
dened the context of the topic, the author identies the current methodo-
logy used for the transition from digital survey to discrete models useful for
structural analysis (Cloud-to-FEM) with a comparison of the dierent variants
(section 2.3.3). In conclusion, reference is made to some exemplary practices
both in the professional world and in research that already include the use of
VPL and more generally of explicit parametric modelling in the eld of structu-
ral analysis (section 2.3.4).
2.3.1 School of thoughts: statistical, mechanistic, holistic
In relation to seismic vulnerability assessment at the urban scale, various re-
search and procedures have been identied that aim to combine expeditious
surveying with accuracy in assessment.
These methods dier mainly in the type of data needed and the accuracy of
the analysis. In particular, an inversely proportional relationship is denoted
between range of applicability and accuracy of the assessment (the more
accurate the assessment, the closer to the architectural scale at the expense
of the urban scale).
Among the most widely used methods for analyzing the seismic vulnerability
of entire territorial districts, there are statistical analyses. These focus on de-
termining vulnerability mainly with reference to several main features of buil-
ding units (such as building/construction type) in order to analyze their distri-
bution over the territory (g. 25). The object of study then becomes an entire
territory without considering the morphological characteristics of individual
urban fabrics (at the cost of accuracy), allowing, however, an expeditious and
broad assessment for the denition of emergency plans covering one or more
territorial areas. The data generally used for this type of analysis are those
contained in national databases such as ISTAT or from territorial bodies and
67
institutions (such as universities or the various sections of the Civil Defense
Department). The advantage of such methods lies in the fact that it is pos-
sible to apply them even in the absence of specic information on building
units, assuming the most probable characteristics through the association of
units with the corresponding building type. It is still possible to increase the
amount of information, thereby rening the analysis at later times. Methods
that refer to this category of analysis include the damage probability matrices
method, the GNDT methodology, and the Risk-UE method.
Fig. 25 | 3D Map - Seismic
vulnerability of residential
buildings in Palermo. Source:
https://coseerobe.gbvitrano.it/
studio-sulla-vulnerabilita-sismi-
ca-degli-edifici-di-palermo.html
Other analyses follow a mechanistic procedure where structural behavior is
investigated in detail by simulating seismic actions on the building unit, from
which limit values of the structure’s strength are derived with great accuracy
(g. 26). Such an investigation requires a considerably higher level of informa-
tion than statistical surveys. Geometric surveys (from the structural scheme
to the internal distribution scheme), analysis of the historical chronology of
interventions, and performance of on-site tests to determine mechanical and
physicochemical characteristics of materials: are just some of the operations
to be carried out in order to accurately determine the value of the seismic
safety of a building. In the presence of historical aggregates, which are com-
plex in terms of typological characteristics and layers of successive interven-
tions, such analyses require more time, energy, expertise, and information
than statistical methods. In spite of this, at present, these analyses are the
most frequently followed procedure for achieving level 2 and 3 assessments
in reference to the 2011 Italian guidelines.
02.BACKGROUND
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 26 | Example of a structural
model composed of macro-ele-
ments. Source: Greco A., Lom-
bardo G., Pantò B. & Famà A.,
(2018): Seismic Vulnerability of
Historical Masonry Aggregate
Buildings in Oriental Sicily, Inter-
national Journal of Architectural
Heritage, pp. 517 - 540
There is also a third way, more related to some schools of thought in the
disciplines of Architectural Restoration and Conservation, which is based on
an analysis by experts through a cognitive process that arranges all the cha-
racteristics of historic buildings. Specically, this type of analysis generally
begins with a survey of the urban growth of the fabric in which the building
analyzed is located, proceeds through the recognition of the construction
apparatus (identifying the construction techniques of the period and thus the
deciencies present), the mapping of deterioration, and the analysis of insta-
bilities. Thereby, such a process leads toward a comprehensive and organic
view of the construction, which is almost completely impossible to describe
through numerical quantities and therefore makes use of descriptive tables
and technical reports. This methodology is also being considered by the 2011
LLGGs, in particular, the reference goes to the level 1 assessment that takes
into account these analyses where the results of these analyses can be qua-
litative (Predari et Al., 2019) (g. 27).
69
Fig. 27 | Analysis of a sample
block in the historic center of
Faenza. From top to bottom:
critical survey of structural com-
ponents, vulnerability and stren-
gth analysis, damage scenario
analysis. Source: Carocci C.F.,
(2013), Conservation of wall
fabric and mitigation of seismic
vulnerability. Introduction to the
study of aggregate buildin-
gs. In: Blasi C. (ed.), Historic
architecture and earthquakes.
Operational protocols for know-
ledge and protection. Wolters
Kluwer Italy, pp. 138-153
2.3.2 Previous studies on seismic
vulnerability for the city of Catania
The city of Catania is located on the east coast of Sicily, right on the slopes
of the volcano Etna. The city is among the most seismically threatened cities
in all of Europe. This particular condition led the city to be repeatedly the
subject of research on seismic risk topics, with several in-depth studies on
the urban scale (Ciatto et al., 2018).
According to its seismic history, there is a 99% probability that an earthquake
with a magnitude greater than 7 will occur in the next 150 years. An earthquake
of this magnitude could repeat the historic peak that occurred in 1693 that re-
shaped the entire city and given the enormous urban growth that has occurred
since then, the repetition of such an event in the present day would create an
apocalyptic scenario in the city in accordance with what has been simulated by
the National Seismic Service. In fact, the number of 'people aected' (dead and
injured) exceeds 160,000 with millions of euros of damage. Also for this reason,
this possible event has been renamed the 'Big One' (Ciatto, 2017).
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Among the main studies conducted over the years, it is worth to mention the
‘Project Catania’. This study has been carried out by the ‘National Group for
the Defense from Earthquakes’ in the 1997 and its aim consisted in evalua-
ting the seismeic vulnerability and safety of the building heritage of Catania.
The researchers adopted a deterministic approach in two dierent scenarios.
The rst simulated the same seismic actions of the soil as in the historical
earthquakes of 1169 and 1693 with an intensity of 10/11 level of the Mercalli
scale and a magnitude of 7.0 - 7.4. The second scenario foresees a weaker
event but with an higher frequency, closely to what happend with another
signicant earthquake in 1818 with an intensity of level 8 (Mercalli scale) and
magnitude of 6.2. The data available for the study were mostly related to
three census surveys. Some of these data were acquired from national da-
tabases, other were acquired directly with groups of social worker led by
some professionals in the recognition of some basic information. A compari-
son of these data highlighed some inconsistency and incongruency with the
data already obtainable from national databases. The study dened two main
categories of building: masonry (historical) and concrete buildings. One of
the main objectives of the project was to compare two dierent approaches
for assessing of large-scale seismic damage. The rst approach, the GNDT
method, uses a statistical method based on the assignment of a vulnerability
score and is widely used in Italy and other European countries. The asses-
sment of vulnerability is carried out in an expeditious manner through sight
analysis, a necessary requirement to have enough data on the whole city.
The procedure is based on the denition of a matrix derived from the 1st
and 2nd level GNDT matrices for the detection of the vulnerability of existing
buildings, whose easily detectable parameters are taken into account. The
survey templates are dierent depending on whether the buildings are with
a masonry or reinforced concrete structural system. The second, more re-
cent and mechanistic approach is based on ‘displacement limit states’. This
methodology, proposed by Calvi in 1999, is based on the evaluation of the
displacement demand in relation to a spectrum. The spectrum depends on
the site conditions and the structural type of the building. According to this
approach, buildings can assume ve dierent situations, corresponding to
values from 1 to 5: substantially intact, slightly damaged, damage extensive,
severely damaged, total collapse. The results obtained through the use of the
two methods are similar, although a direct comparison is complicated due to
the dierent nature with which they were developed. The results have been
stored in a GIS enviroment so that these ones could be consulted (Faccioli,
2000; Liberatore et al., 2000).
Another important study is the project ‘Risk-UE’, which started in 2001 and
covered three years under the supervision of the European Commision with
the aim to dene “an advanced method for earthquake risk scenarios with
applications to dierent European cities”. Catania was included, with other
six cities, under this ambitious research project. The choice of Catania was
made taking into account the availability of data, especially coming from the
71
‘Project Catania’ few years before. The project divided the city with referen-
ce to the constructive system of the buildings (masonry, concrete, steel and
wood). The earthquake scenarios used for the analysis were obtained from
the analysis of the seimeic history of the territory. For this reason the scena-
rios were the two same adopted in the ‘Project Catania’. The project took
into account the city’s exposure by identifying four periods of interest (day
crisis, night crisis, normal and recovery) and their exposure in accordance
with the denition of homogeneous areas and parameters such as popu-
lation, urban space, functional activities and services, urban activities, go-
vernment activities, identity and culture and external radiance. The project
denes dierent vulnerability indices with respect to the dierent objects of
the analysis. There are therefore dierent methods of analysis with respect to
residential buildings, historic and monumental buildings and infrastructure.
With respect to residential buildings, the vulnerability index is established
through the denition of a parameter ranging from 0 to 1. The calculation is
the addition of predetermined indices related to the building type, a regional
modier and a behaviour modier that depends on the characteristics of the
building. These indices are determined on the basis of statistical information
and collected through national survey campaigns. A mechanistic approach is
only partially introduced for historic and monumental buildings. Indeed, the
analysis of these architectures is based rstly on a territorial level taking into
account the building type (statistical approach) and in the second part there
is an analysis of a mechanical type. Using the earthquake response spectrum
as the seismic input, capacity curves are dened that express the vulnerabili-
ty of the building. However, this type of analysis concerns only 150 buildings
selected for analysis, including many churches and palaces that are generally
well-made with respect to the techniques used in their respective historical
time periods (Faccioli, 2003).
The aforementioned research works brought national and international at-
tention to the city of Catania, seeking to gradually integrate an approach that
from being statistical and qualitative on the basis of visual surveys at the
discretion of the operator’s experience, became increasingly scientic and
analytical through a mechanistic approach. Unfortunately, this is no guaran-
tee of a correct result compared to the other methods used, but it does make
it possible to normalise the type of analysis and knowledge gathered from the
urban environment by reducing the subjectivity of the operators and errone-
ous statistical trends.
The lines of research that have continued in this direction until nowdays ad-
dressed the question of how to increasingly integrate mechanistic approa-
ches to the needs of expeditious surveying considered on a highly complex
historical building heritage. In relation with the aim of this thesis, from the
analysis of the state of the art has emerged a specic work that proposes an
expeditious methodology aimed at structural modeling that allows the analy-
sis of seismic vulnerability using mechanistic methods. This research project
02.BACKGROUND
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
falls within the University of Catania FIR 2014 University Research Funding
Program and is entitled ‘Seismic Vulnerability of Historic Aggregate Buildings.
New structural modeling and expeditious methodologies and approaches.’
The research was conducted by a multidisciplinary team and focused on se-
veral residential areas in the historic urban center of Catania. In this work, a
typological study was conducted that led to the identication of 9 distinct
building types based on the amount and mode of accretion of a primary wall
cell. The typologies identied were summarized into 9 archetypes used as a
reference for the construction of geometric models. These models were then
used to dene the corresponding structural models within the software that
allows the simulation of seismic actions on structures (in accordance with the
procedure for ‘macro-elements’). Finally, a seismic vulnerability assessment
was conducted both on individual archetypes and on an ideal urban aggregate
obtained by assembling several archetypes (Caddemi et Al., 2018) (Fig. 28).
Fig. 28 | From top to bottom:
archetypal one-storey house,
typological classification of
building units in an aggre-
gate of the historic center of
Catania, (bottom left) geome-
tric model and structure of an
ideal representative aggregate,
(bottom right) Capacity curves
of structural units within the
aggregate (average masonry
quality) in longitudinal (a) and
transverse (b) directions. Sour-
ce: Greco et al., 2018.
73
2.3.3 Cloud-to-FEM methods: BIM and NURBS modeling
There are research threads dedicated to the optimisation of digital workows
in an attempt to use semi-automatic and automatic methods to obtain geo-
metric models useful for structural analysis. The topic spans two disciplines,
digital surveying and structural analysis, which have apparently dierent re-
quirements both conceptually and in terms of type, accuracy and quantity of
data. Therefore, dening a valid approach for both means bringing together
both representation and analysis necessities. In the national and international
sphere, there are various researches that have attempted to bridge this gap in
research by dening dierent approaches that are useful to take into account
with respect to the experiments presented in this thesis. Given the vastness
of the topic, the following is a selection of works that may be representative
of the approaches currently found in the literature.
Concerning the transition from digital models to geometrical entities useful
for FEA there are dierent approaches proposed in the last years. The evo-
lution of geomatics methodologies allowed to acquire structures with high
accuracy and to integrate in the same pipeline of work the application of BIM
methodologies towards the denition of nite elements models. This proce-
dure is known in literature as Scan-to-FEM (also Cloud-to-BIM-to-FEM) ap-
proach (Barazzetti et al., 2015; Dore et al., 2015) (g. 29).
Fig. 29 | From left to right:
NURBS geometries produced
in the BIM environment and
converted into mesh models for
FEM analysis. Source: Dore et
al., 2015.
The transition from BIM to FEM contemplated in these methodologies does
not always occur straightforwardly despite the interoperability of BIM models.
Moreover, FEM analyses are not univocally dened by a single model, but
they vary with regards to analysis needs (Abbate et al., 2020). Other methods
do not take a BIM approach and move from point cloud to manual NUR-
BS modeling. This procedure articulates in the following steps: generation
of polygonal model from point cloud, extraction of outlines coming from the
slices of the model, construction of NURBS model on the basis of sections
and discretization of NURBS model into mesh model for analysis in dedica-
ted FEM software (Fortunato et al., 2017; Fang et Al., 2021). In some scena-
rios, the cloud would not be cross-sectioned, but after the cleaning phase of
the non-structural elements, it would be converted into a closed mesh trying
02.BACKGROUND
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
to obtain a mesh as smooth as possible. Most often, the mesh consists of
triangular faces in order to guarantee the atness of all elements. This mesh
is then transformed into a closed NURBS polysurface within mathematical
modelling software (e.g. Rhinoceros). This intermediary operation is occasio-
nally necessary since some structural analysis software takes a closed solid
polyhedron model as input. Eventually, the model provided as input to the
FEA software recomputes a mesh from the mathematical model provided as
input in order to optimise faces and densities with respect to the analysis to
be conducted (Quattrini et Al., 2019) (g. 30).
Fig. 31 | Generation of a mesh
model for FEM analysis from a
NURBS mathematical model
generated using point cloud
cross sections. Source: Fortu-
nato et al., 2017.
In the literature it is possible to nd experiences of process automation that
aim to save time and overcome the criticalities present in BIM models expor-
ted for FEM analysis (mesh compatibility, local deformations and too small
details) (Castellazzi et al., 2015). It is worth to mention that there are not many
applications towards the adoption of computational design tools for optimi-
sing and accelerating the Scan-to-FEM pipelines (Funari et al., 2021). Another
research gap can be found in the lack of applications between city blocks
acquisition-modeling and a semi-automatic transition to FEA (g. 31).
Fig. 30 | Parametric Scan-to-
FEM models. From left to right:
original point cloud, genera-
tion of the parametric model,
discretization into mesh model
and FEM analysis. Source:
Funari et al., 2021.
75
2.3.4 Parametric and VPL based related
works on structural analysis
The introduction of digital workows from reality to structural analysis has
made it possible to integrate parametric modelling solutions into the structu-
ral analysis phase itself. This phenomenon started especially in workows
that included HBIM methodologies. Indeed, this methodology often exploits
the potential of VPLs to recreate components that are dicult to reproduce
manually. The interoperability of BIM models with VPLs has made it possible
to exploit the latter not only for modelling purposes but also for performing
proper analyses. This is due both to the possibility of developing textual co-
des within VPLs and also due to the existence of structural engineering plu-
gins that can directly interface with the models created (as already described
in section 2.2.3). The BIM-to-FEM workows that develop this approach ini-
tially involve a survey and study phase of the building, then BIM modelling
through the point cloud-based architectural model, and nally the structural
and analytical model. From the BIM environment, the analytical model is ex-
tracted using VPLs that are capable of receiving geometries and attributes of
each individual component. Within the VPL environment is possible to dene
loads and boundary conditions. At this point, two scenarios can occur. In the
rst, the VPL is exploited to create the input model to an external analysis
software (through the creation of a le or directly with a dedicated plugin),
while in the other scenario, the analysis can take place directly within the VPL
environment (through expressly dened codes or via specic plugins). Even-
tually, the results obtained are processed by VPL by means of which the BIM
model is updated with the result of the analysis (Croce et Al., 2022; Croce et
Al., 2021; Calvano et Al., 2022; Massafra et Al., 2020; Moyano et Al., 2022)
(g. 32).
In other cases, the use of VPL is useful for improving the congurations of
parameters and families within BIM environments, so that models can be ca-
librated to directly obtain information such as the quality index of the masonry
automatically through the input of appropriate indices. This approach makes
the structural analysis even more intrinsic, developing it already during the
HBIM modelling and information enrichment phases (Calvano et Al., 2022;
Massafra et Al., 2021) (g. 33).
02.BACKGROUND
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3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 33 | The model generation
procedure and grasshopper al-
gorithm. (a) TLS point cloud; (b)
Point cloud segmentation; (c)
Vectorised cross-sections; (d)
Ideal Plane; and (e) TLS point
cloud-Ideal Plane comparison.
(Source: Massafra et al., 2021).
Fig. 32 | Comparison between
Seismic analysis workflow CAD
based and BIM based. Source:
Croce et al., 2022.
77
2.4 From 3D City Models to City Information
Modeling
To conclude, this section introduces the topic of 3D city models. This subject,
which is one of the main topics in relation to the aims of the thesis, was left for
last as it is transversal to all subjects discussed and contains several points
of contact with them. In particular, the denition and standards for 3D city
models are introduced (section 2.4.1). This is followed by a description of City
Information Modeling (section 2.4.2), the methodological paradigm in which
the thesis work is embedded. Finally, a review is provided regarding the state-
of-the-art of current applications in relation to City Information Modeling that
are close to the topics of the research work (section 2.4.3).
2.4.1 Denition, standards and applications of 3D City Models
Currently, a 3D City model can be dened as “a digital representation, with
three-dimensional geometries, of the common objects in an urban environ-
ment, with buildings usually being the most prominent objects” (Arroyo Ohori
et Al., 2022). This virtual representation is usually adopted to store, visualize
and interact with digital urban data acquired from reality that include terrain,
building, vegetation as well as roads and transportation systems models. Vir-
tual 3D city models’ ability to visually integrate diverse geoinformation into a
unied framework is one of its distinguishing features. As a result, they enable
the creation and management of complex urban information environments
(Döllner et Al., 2007; Billen et Al., 2014; Zhu et Al., 2009). The generation of 3D
city models can take place from dierent types of acquisition and data. Exam-
ples of methods through which 3D city models can be generated are: pho-
togrammetry (terrestrial and aerial), laser scanning, extrusions from 2D cad,
CAD/BIM model conversion, procedural modelling, crowdmapped opendata,
etc. Generally, 3D city models are dened with respect to a data structure
and format according to the type of datasources available, the expertise of
those producing them and the type of output expected. The applications of
3D city models cover almost all disciplines, so their generation and mana-
gement is a topic of considerable scientic relevance. In Biljecki (2017), 29
dierent applications of city models are identied. These include analyses for
solar irradiance estimation, energy demand estimation, inhabitant estimation,
visualisation of the urban environment for navigation systems, visibility analy-
sis, shadow studies for urban climate analyses, applications for land registry,
urban planning, facility management, emergency response, etc.
In the context of this research, semantic 3D city models will be discussed.
The motivation for semantic 3D city models is the fact that they allow infor-
mation to be extracted from the city model (e.g. how many inhabitants are
there in a city block? or what are the years and construction techniques of its
buildings?, etc). Models that do not allow queries and/or interactions cannot
be called semantic 3D city models but 3D representations of a territory (a
mesh obtained from photogrammetry of an urban area cannot be dened as
a semantic model since the computer does not know how to extract this in-
formation). These digital artefacts are data models where the relevant objects
02.BACKGROUND
78
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
To conclude, this section introduces the topic of 3D city models. This subject,
which is one of the main topics in relation to the aims of the thesis, was left for
last as it is transversal to all subjects discussed and contains several points
of contact with them. In particular, the denition and standards for 3D city
models are introduced (section 2.4.1). This is followed by a description of City
Information Modeling (section 2.4.2), the methodological paradigm in which
the thesis work is embedded. Finally, a review is provided regarding the state-
of-the-art of current applications in relation to City Information Modeling that
are close to the topics of the research work (section 2.4.3).
2.4.1 Denition, standards and applications of 3D City Models
Currently, a 3D City model can be dened as “a digital representation, with
three-dimensional geometries, of the common objects in an urban environ-
ment, with buildings usually being the most prominent objects” (Arroyo Ohori
et Al., 2022). This virtual representation is usually adopted to store, visualize
and interact with digital urban data acquired from reality that include terrain,
building, vegetation as well as roads and transportation systems models. Vir-
tual 3D city models’ ability to visually integrate diverse geoinformation into a
unied framework is one of its distinguishing features. As a result, they enable
the creation and management of complex urban information environments
(Döllner et Al., 2007; Billen et Al., 2014; Zhu et Al., 2009). The generation of 3D
city models can take place from dierent types of acquisition and data. Exam-
ples of methods through which 3D city models can be generated are: pho-
togrammetry (terrestrial and aerial), laser scanning, extrusions from 2D cad,
CAD/BIM model conversion, procedural modelling, crowdmapped opendata,
etc. Generally, 3D city models are dened with respect to a data structure
and format according to the type of datasources available, the expertise of
those producing them and the type of output expected. The applications of
3D city models cover almost all disciplines, so their generation and mana-
gement is a topic of considerable scientic relevance. In Biljecki (2017), 29
dierent applications of city models are identied. These include analyses for
solar irradiance estimation, energy demand estimation, inhabitant estimation,
visualisation of the urban environment for navigation systems, visibility analy-
sis, shadow studies for urban climate analyses, applications for land registry,
urban planning, facility management, emergency response, etc.
In the context of this research, semantic 3D city models will be discussed.
The motivation for semantic 3D city models is the fact that they allow infor-
mation to be extracted from the city model (e.g. how many inhabitants are
there in a city block? or what are the years and construction techniques of its
buildings?, etc). Models that do not allow queries and/or interactions cannot
be called semantic 3D city models but 3D representations of a territory (a
mesh obtained from photogrammetry of an urban area cannot be dened as
a semantic model since the computer does not know how to extract this in-
formation). These digital artefacts are data models where the relevant objects
79
Fig. 34 | A building is semanti-
cally decomposed into different
objects, and each object is
defined with geometry. (Source:
Arroyo Ohori et al., 2022).
(and their components) are structured with respect to a hierarchy and have
attributes linked to them. These models are a collection of objects belonging
to dierent classes (building, road, bridge, tree, etc.). Each object possesses
at least one geometric representation and may also possess attributes. In
addition, each object is decomposed into other homogenous parts, each of
them with a geometric representation and attributes. Taking an object belon-
ging to the building class as a reference, it is decomposable into walls, oors,
windows, roof, etc. For this reason, a 3D city model is dened spatio-seman-
tically coherent if there is an univocal relationship of each decomposed ele-
ment with its host object, both geometrically and semantically. Conceptually,
these models are structured in much the same way as BIM models which rely
on families and parameters (as described in section 2.2.1) (Arroyo Ohori et
al., 2022) (g. 34).
Due to the great variety of types of 3D city models, it became necessary to
dene an international standard that would dene the data structure from a
semantic point of view so that even if models were obtained from dierent
data and processes, they would all be constituted in the same way. The mo-
deling standard for three-dimensional information models of cities and urban
systems is the CityGML 2.0 from the Open Geospatial Consortium, which
identies ve levels of detail (LOD) in the three-dimensional representation
from 0 (footprint on the ground) to 4 (building modeled both internally and
externally) and uses a set of classes to describe city characteristics. In the
latest version of this standard, there is also mapping of IFC elements (from
BIM models) to the CityGML classes (OGC CityGML 3.0 Conceptual Model)
recognized by the Open Geospatial Consortium (OGC) and the ISO/TC211
technical committee. It is worth pointing out that these levels of development
correspond to geometric and semantic features requirments informational re-
quirements. The dierent levels of detail are discussed in more detail below.
LOD 0 - It is the two-dimensional representation of the footprint of buildings
on the ground through polygons. It can also contain polygons relating to the
projection of roof edges in order to facilitate the transition from 2D to 3D. It
can also be in the form of 2.5 D digital terrain model (DTM), since it represents
02.BACKGROUND
80
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 35 | The five LoDs in
CityGML for the exterior of a
building. (Source: Biljecki et al.,
2014).
the topography of the territory under analysis on which the aerial photo is
projected. In this case, the buildings are represented as 2D ground footprints
projected on a 3D surface. This level is widely adopted in GIS application for
territorial/country scale models.
LOD 1 - LOD 1 consists of the representation of building type objects in the
form of prisms obtained from the extrusion of the LOD0 footprints on the
ground. This level provides the volumetric characteristics of the urban envi-
ronment at the scale of a city or settlement.
LOD 2 - This level provides a model capable of representing roof geometry
with simple shapes as well as semantically breaking down buildings into their
dierent subclasses (e.g. walls, roof, oors). This level reaches the scale of
the neighbourhood, allowing for more detailed analyses due to the presence
of roof geometries. Furthermore, thanks to the semantic decomposition into
sub-components, this level can be said to reach the ‘BIM scale’ where each
building component is related to the others and has geometry and attributes.
LOD 3 - LOD3 increases the detail compared to LOD2 by adding all the ope-
nings present on the envelope (doors and windows) plus other objects not
comparable to LOD2 such as chimneys and roof details. The scale of this
level is architectural, from 1:200 onwards.
LOD 4 - With LOD 4, the highest level of detail is achieved. In addition to the
features presented in LOD3, the dimension of the interior space is added, so
the LOD4 can present elements such as oors, interior walls, doors, stairs
and furniture (Biljecki et Al., 2014; Biljecki et al., 2013) (g. 35).
The predominantly geometric nature of LODs is due in part to that the appli-
cations of 3D city models generally have requirements more related to geo-
metry than to object semantics. While as far as information is concerned, it
is often layered two-dimensionally through the use of GIS without a precise
location. However, applications involving 3D city models have scaled down to
the architectural scale requiring greater precision and more advanced mana-
gement of collected geometries and attributes. Furthermore, the impossibility
of some digital acquisition techniques in particular urban environments has
81
Fig. 36 | Two variants of LOD2
and an LOD1 model exposing
the shortcomings of the CityG-
ML LOD concept, and why the
computer graphics principles
cannot be fully applied to GIS
and 3D city modelling. . (Sour-
ce: Biljecki et al., 2016).
highlighted certain drawbacks that the adoption of such LODs raises during
the modelling phase (Machl, 2013; Löwner et Al., 2016; Biljecki et Al., 2016).
Below is an example of some of the ambiguities involved in such LODs. In-
deed, in gure x, it can be seen that the rst and second models from the left
correspond to LOD2. However, the rst is more accurate as it is the result of
an aerial and terrestrial accquisition that allowed the exact position of the roof
and walls to be dened, while the second is obtained from the projection of
the roof edges alone, leading to an erroneous position of the walls. Another
example is the third model, where a simple addition to the building with a
dierent height is extruded up to the maximum height (g. 36).
These ambiguities also arise from the technological advancement of acqui-
sition and modelling techniques, which sometimes only allow for a portion of
the acquired reality or already prepared and dened 3D information models.
This issue is amply discussed in the literature (Guercke et al., 2011; Fan et al.,
2012; Deng et al., 2016). According to Biljecki et al. (2016), one of the main
problems with these ambiguities is the lack of standards that relate acquisi-
tion techniques to the models obtained, thus generating confusion over the
use of nomenclature. How should be classied a building that is represented
only by a prism (LOD1) but has surfaces inside it that represent oors (LOD4)?
To answer these type of questions, the 3D Geoinformation Research Group
at the University of Delft proposed a review of LODs. This revision consists of
16 LODs obtained by dening 4 versions for LODs ranging from 0 to 3. LOD4
is excluded, as for urban applications it is currently not used much due to the
diculty of acquiring data concerning the interior of buildings (privacy issues)
(Biljecki et Al., 2016). Below is the matrix with the proposed new LODs (g.
37).
02.BACKGROUND
82
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 37 | Visual example of the
refined LODs for a residential
building proposed by Biljecki et
al. (2016). (Source: Biljecki et
al., 2016).
This LOD allocation has also been formalised as the reference standard for
the CityJSON format (recognised by the OGC), which will be introduced later.
Among the various changes introduced with this LOD perspective, it is worth
highlighting, with respect to the applications presented in this thesis, those
concerning LOD3. Generally speaking, the acquisition of LOD3 is a very la-
borious process that combines dierent techniques. Generally, there is both
terrestrial (photogrammetry or laser scanner) and airborne (photogrammetry
or LIDAr) acquisition. For this reason, this level does not cover vast areas but
concentrates on the size of the neighbourhood. Compared to the matrix pre-
sented in gure x, LODs 3.2 and 3.3 are intended to dierentiate the level of
detail of the objects in the envelope (LOD3.2 >1.0 m, LOD3.3 >0.2 m). These
levels permit better classication of models obtained from BIM environments
83
or modelled for visualisation purposes. However, some models in the litera-
ture (Franić et al., 2009; Novaković, 2011) have dierent LOD characteristics.
Generally, such LODs, referred to as hybrids, result from dierences obtained
in terrestrial and/or aerial acquisition (or lack of one of them). For this reason,
LOD3.0 corresponds to a model with a greater detail of roof geometries than
LOD2, but in other characteristics it may fall within LOD2 (greater quality in
aerial acquisition). On the other hand, LOD3.1 emphasises greater quality in
terrestrial acquisition, taking into account that this type of acquisition can
achieve the requirements of LOD3 except for the roof, which is often out of
range (Biljecki et Al., 2016).
The ocial format recognised by OGC through which 3D city models can
be encoded, developed and shared is CityGML with its three encodings (or
sub-format) which are: XML encoding (usually called ‘CityGML’), CityJSON
and 3DCityDB (a database schema for PostgreSQL). The last one is not of-
cial, however is largely used around the world (Arroyo Ohori et Al., 2022).
CityGML was released in 2008 in its rst version. It consists of a data mo-
del that enables the representation of semantic 3D city models. The latest
version is 3.0 released in 2021. CityGML allows a description of buildings
both internally and externally, in accordance with the original ve LOD levels
described above. The format is broken down into several modules, each of
which can be described textually and via UML diagrams (Open Geospatial
Consortium, 2012) (gg. 38, 39).
Fig. 38 | The modules of the
CityGML data model. (Source:
Arroyo Ohori et al., 2022).
02.BACKGROUND
84
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 39 | Overview of the UML
model for the core of CityGML.
(Figure © 2021 Open Geospa-
tial Consortium, Inc.) (Source:
Arroyo Ohori et al., 2022).
CityGML is encoded using GML (Geography Markup Language) via XML.
Therefore, a CityGML le consists of an XML text le describing parts of the
3D city model. Unfortunately, this method of writing the le is not among
the most accessible, as XML encoding is not human readable with a rigid
hierarchical structure and hardly adapts to the web. This has reduced the
applications of CityGML in other software, determining a reduction of the
attention towards the standard itself, thus causing a proliferation of semantic
3D city models that are hardly interoperable in terms of both le and semantic
structure (g. 40).
Fig. 40 | Part of a CityGML file
containing 2 buildings. (Source:
Arroyo Ohori et al., 2022).
85
For this reason, CityJSON was developed. It presents itself as an alterna-
tive to XML encoding that aims to be more user-friendly and thus improve
dissemination thanks to the API support it enjoys. The current version of Ci-
tyJSON is 1.1.2, released in August 2022. The adoption of JSON encoding
makes this format accessible to most programmers, especially beginners. In
fact, JSON les are structured from the nesting of common dictionaries and
thus nd easy development in both existing software and new applications.
In CityJSON, the hierarchical structure is reduced to a minimum, through a
division between rst- and second-level CityObjects (g. 41). All hierarchical
relationships are explicitly stated within the objects in the form of attributes
(“parents” and “children”) and not by means of an actual semantic structure.
Thus, by using the appropriate keys, the data model can be reconstructed
correctly (Ledoux et a., 2019).
Fig. 41 | The implemented
CityJSON classes (same name
as CityGML classes) are divided
into 1st and 2nd levels. (Sour-
ce: Arroyo Ohori et al., 2022).
2.4.2 City Information Modeling: beyond GIS and BIM
As discussed in section 2.4.1, 3D city models currently share a format that
guides the denition of the semantics of city objects. However, the multitude
of methods used for acquisition and modelling have made it dicult to dene
a methodological paradigm as has been the case for GIS and BIM systems.
In the state of the art, however, a method has emerged that has become an
increasingly common subject in the literature for several years now, due in
part to the debate about the digital revolution in Smart Cities. It is called City
Information Modeling and is considered to be the synthesis of several wor-
kows in the branch of parametric information modelling.
02.BACKGROUND
86
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
The denition of City Information Modeling is an issue that has been widely
debated internationally in recent years (Simonelli et al., 2018). An outlining
according to information modeling standards is at least complex given the
hybrid nature of the BIM and GIS environment. Therefore, it is necessary to
identify what characteristics allow a CIM model to be dened as opposed to
a 3D GIS or BIM extended to the urban scale. In agreement with Xue, F. et al.
(2021), the meaning that the 'I' of 'Information' takes on in CIM versus GIS
and BIM provides the correct key.
According to one of its rst denitions, it is called the Urban Information Mo-
del which “integrate the multidimensional urban aspects like economy, so-
ciety and environment with 3D urban model plus temporal dimension. Urban
information model will provide comprehensive information support to various
urban planning application systems” (Hamilton et al., 2005). One of the most
established and recognised denitions in the literature is that a City Informa-
tion Model consists of a system of urban elements represented by 2D and 3D
elements containing information, linked by semantic relationships (Stojano-
vski, 2013). Therefore, we talk about City Information Modeling when we refer
to the technologies and practices used to develop CIM models and to exploit
their potential for urban and spatial analysis (Xue et al, 2021; Mozuriunaite et
al., 2021) (g. 42).
We can think of CIM models as BIM systems applied to urbanism, a 3D se-
mantic expansion of GIS models (Stojanovski, 2013). As Xu et al. highlight
(2014), the major diculty in CIM is the information modeling since these mo-
dels deal with outdoor and indoor data semantically linked. It is worth to men-
tion that 3D city models developed on BIM environments and then visualized
in GIS (and vice versa) cannot full the denition of CIM since geometries and
information are not able to modify and interact to each other beyond visua-
lisation and queries purposes. This type of 3D city models, also known as
‘GeoBIM’ (Noardo et al., 2020), is mainly focused on interoperability between
BIM and GIS standard formats (IFC and CityGML) (Sun et al., 2020).
Fig. 42 | Concept diagram of
the definition of City Information
Modeling. (Source: Mozuriunai-
te et al., 2021)
The rst developments of CIM can be identied in applications aimed at over-
lapping point clouds obtained through instrumental surveys (laser scanning,
photogrammetry) and geo-referenced 3D models obtained in a GIS environ-
ment (Julin et al., 2018) (g. 43).
87
Fig. 43 | View of the three-di-
mensional information system
Helsinki 3D+. It is possible to
consult the system both as a
CIM (created from geodata,
BIM models and laser scanner
point clouds) and as a 'Reality
Mesh model' (polygonal pho-
togrammetric model obtained
from aerial photographs of the
entire city). Source: Helsinki
3D+ Information Model. Link:
https://kartta.hel.fi/3d/
In these models, information was (and still is) stratied by layers within one-
and two-dimensional geometries (Goodchild 1991; Lu et al., 2018). The rst
substantial dierence between GIS and CIM emerges: the latter considers
urban scenarios such as ow analysis, energy simulations, and construction
techniques that relate to each other through cause-and-eect relationships
(similar to what happens in BIM) and not only by the adjacency of georefe-
renced locations. A 3D (semantically dened) model developed within GIS
software can thus be considered a specic subcategory of CIM (Xu et al.,
2014a; Liu et al., 2017).
As for BIM, it concerns all the information at the architectural scale that is fun-
ctional for the design, management, and preservation of a given architecture
(Lo Turco, 2015; Osello, 2015). In CIM, however, the relationships between
the individual architectural unit and the information related to the three-di-
mensional urban context intervene (Xu et al., 2014a). Based on these relation-
ships, in CIM, the semantic enrichment of the architectural units themselves
is dened (e.g., the identication of the main front with respect to the street).
Currently, one of the challenges in City Information Modeling is the integration
of Geographic Information Systems (GIS) and Building Information Modeling
(BIM) into a single model that allows for the investigation of dierent scales of
detail (in terms of geometry and information) in a given urban area for dierent
purposes (Ying et al. 2020; Jovanovic et al. 2020). Thus, there are on the one
hand systems that implement data describing the area and/or the city predo-
minantly in the two dimensions, and on the other hand systems that describe
in detail, both in terms of geometry and information, the individual building
and its construction components.
In Mozuriunaite et al. (2021), the dierent stages of evolution of CIM systems
are described in four stages. In the rst phase, there is the combination of GIS
and digital modelling technologies with the aim of creating a single, coherent
urban information model (such as GeoBIM models). The second phase deals
with improving the interaction with these models through the introduction of
02.BACKGROUND
88
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
technologies such as Augmented and Virtual Reality in order to allow more
eective consultation, both on-site and in the oce, even for non-experts.
The third phase, on the other hand, consists of the integration of the Internet
of Things, in order to embrace the concept of a city-wide Digital Twin. The
fourth phase, the one whose evolution is currently in progress, consists of
the application of AI mechanisms in order to further improve the creation,
management and maintenance of the CIM, reducing costs and increasing the
quality of output (g. 44).
Fig. 44 | Stages of CIM’s
development and application.
(Source: Mozuriunaite et al.,
2021)
2.4.3 3D City Modelling signicant case studies close to
CIM for seismic risk assessment and management
Nowadays, there is a wide range of CIM applications covering dierent di-
sciplines (Xue et al., 2021; Xu et al., 2014). It is possible to distinguish three
main approaches for developing CIM models: bottom-up, top-down and pa-
rametric.
The rst one (or BIM-based) focuses more on remote sensing on site ac-
quisition (close- and mid- range laser scanning and photogrammetry) with
subsequent manual and semi-automatic modeling processes in BIM and
CAD environments (Pelliccio et al., 2017; Zhang et al, 2021; Avena et al.,
2021; Parrinello et al., 2020). These procedures often merge BIM and GIS
data enabling the users to make queries and display models on web-based
platform. In these models Computational Design is applied, through Visual
Programming Languages (VPL), to link, sort and merge metadata between
models and environments but not for modeling purposes (g. 45).
The top-down procedures deal mainly with long-range remote sensing tech-
niques (e.g., Airborne LiDAR data) and geodata (coming from online open-da-
ta sources or datasets held by local institutions) which are further develo-
ped inside GIS-based procedural modeling digital environment (Biljecki et
al, 2015; Nys et al., 2020; Wang et al., 2018, Pârvu et al., 2018). Top-down
models usually don’t need any further integration (unlike bottom-up models)
with exception for indoor data that are inserted via the conversion of IFC les
into CityGML objects (Biljecki et al, 2021). These models are closer to the
89
Fig. 45 | Example of BIM-ba-
sed 3D city model for risk
assesment. The methodology
applied was called Historical
Town - BIM. (Source: Pelliccio
et al., 2017)
denition of CIM since they are based on CityGML standard where the city
is treated as a whole system composed by dierent objects with geometries
and metadata (OGC CityGML 3.0, 2012). In these models, CD is applied by
using traditional textual programming languages for creating algorithms that,
starting from point clouds segmentations, allow to obtain building geome-
tries. The development paradigms for CIM presented so far are very expensi-
ve in terms of technologies and expertise needed. Therefore, they are not su-
stainable except for large cities, leaving small and medium centres excluded
from the potential utility of CIM for emergency management (g. 46).
Fig. 46 | CityJSON Building
Generation from Airborne Li-
DAR 3D Point Clouds. (Source:
Nys et al., 2020)
In this context there is a third approach used for generating CIM model often
called ‘parametric urbanism’ (De Jesus et al., 2018). This approach is cha-
racterized by Computational Design workows that often interoperate with
open-data and remote sensing products. The main work environments are
VPLs connected with CAD software. In particular, the VPL Grasshopper, thanks
to several plugins dedicated to 3D city modeling, has supported the develop-
ment of several research activities related to the CIM paradigm (De Jesus et al.,
2018; Calvano et al., 2019; Fink & Koenig, 2019; La Russa & Genovese, 2021).
The parametric approach relates to the previous ones regarding responsive-
ness between les of dierent nature (ex. IFC and SHP), interaction with digital
survey products and standards for 3D city models (CityGML) (g. 47).
02.BACKGROUND
90
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
As regards the Italian scenario, Italy’s architectural heritage has been the fo-
cus of study, documentation and research campaigns for the design of plans
of measures aimed at risk assessment and mitigation. Several studies have
been conducted aimed at the creation of integrated systems for documen-
ting historic centers in order to implement conservation and transformation
policies (Parrinello et al., 2019), in which the issue of interoperability between
GIS and BIM becomes fundamental (Cecchini, 2019) or addressed to the cre-
ation of a methodology for the management of prevention and reconstruction
related to natural disasters (Calvano et al., 2019). These strategies can be
divided according to the approach to modeling from the architectural to the
urban/territorial scale and vice versa. Thus, there is a top-down approach
(treatment of geodata for the development of primitive geometries developed
up to the architectural scale) and a bottom-up approach (on-site instrumental
surveys with subsequent reverse-modeling of the surveyed objects with the
progressive abstraction of the geometries). This also determines the ease of
the creation of information databases to be linked to CIM. In fact, in top-down
processes, it is assumed that there is an upstream source of geodata through
which to initiate modeling by semantically dierentiating the geometries pro-
duced while in the bottom-up approach the information database is obtained
downstream by rst having to segment the geometries obtained from the
initial instrumental survey.
The choice of these approaches is related to the objectives of the applica-
tions. For example, it may be necessary for the short term to possess a point
cloud functional for visual inspections that determine the start of a bottom-up
workow. Some case studies carried out by Italian research groups are pre-
sented below. Both approaches can be found in the works identied although
there is always a higher tendency toward one of the two, even if only at the
methodological level.
In this context, the project promoted by the joint LS3D Landscape, Survey
and Design (University of Florence) and Dada Lab (University of Pavia) labora-
tories, having as object of study the city of Pavia, dealt with the creation of di-
Fig. 47 | 3D reconstrution
of the city of Amatrice using
Grasshopper and opendata.
(Source: Calvano et al., 2019)
91
Fig. 48 | 3DBethlehem. Urban
growth management and
control for heritage develop-
ment and life improvement in
the city of Bethlehem. (Source:
Parrinello et al., 2020)
gital databases for the preservation of urban heritage (De Marco et al., 2018).
Further research conducted by the Pavia group responds to the need to de-
ne operational strategies for the experimentation of documentation systems
useful for the development of tools for the management and enhancement of
historical heritage. In particular, the research is part of an Italy-Israel interna-
tional cooperation project for city knowledge (Parrinello et al., 2019) (g. 48).
02.BACKGROUND
92
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
It is also worth highlighting the procedures adopted by local institutions spre-
ad across the territory to counter seismic risk historic cities. Generally, these
procedures are based on the compilation of geodatabases useful for establi-
shing risk exposure values for urban centres but also for managing the post-e-
mergency scenario. In these activities, territorial institutions face the diculty
of being able to populate these geodatabases with data that can be upda-
ted over the medium and long term. For this reason, opendata have been
increasingly considered by public institutions in recent years. Indeed, the use
of open source data such as geographic-spatial data from OpenStreetMap
(OSM) can facilitate this process. In this direction, the Civil Defense’s ERIKUS
system (Regione Piemonte, 2018) (g. 4) also made use of data from OSM, a
crowd mapping platform used by so-called ‘digital humanitarians,’ volunteers
who provide digital mapping data. In 2010 following the Haiti earthquake, the
Humanitarian OpenStreetMap Team (HOT) was formed to coordinate volunte-
er mappers during crisis scenarios or in support of specic mapping projects
(Minghini et al, 2017). Even during the last dramatic earthquakes in Italy, L’A-
quila 2009 and central Italy 2016, and the most recent explosion in the port
of Beirut (OpenStreetMap Wiki, 2020), OSM communities helped collect and
produce data (g. 50).
If the experiments of the Pavia group are more oriented toward the develop-
ment of 3D GIS systems in the eld of architectural heritage management
in urban contexts, which is useful for a mapping of the territory in a pre-e-
arthquake scenario, the METRICS project of the Built Heritage Innovation
Laboratory of CNR-Itabc shifts the focus to the documentation and descrip-
tion through BIM systems of sites aected by the earthquake. It constitu-
tes the rst industrial research project to develop innovative methodologies
and technologies for sustainability and safety in historic centers (METRICS,
2021). Also developed in a BIM environment, the methodological proposal of
HT_BIM (Historical Town Building Information Modeling) for risk analysis in
historic centers (Pelliccio et al., 2017) extends established practices in BIM
digitization of historic heritage to the urban scale (g. 49).
Fig. 49 | METRICS project of
the CNR-Itabc Built Heritage
Innovation Laboratory. (Source:
METRICS., 2021)
93
Fig. 50 | Operational interfa-
ces of the GIS-based ERIKUS
system. Source: "ERIKUS the
earthquake damage census.
An overview of what is possible
with GFOSS tools" by Luca
Lanteri, Erika Ceriana Mayneri
and Stefano Campus during
QGIS Day 2019, Florence.
In the literature there are applications that use data from OSM to create city
models that visualize environmental data and allow simulations to be ope-
rated (Hadimlioglu and King, 2019). However, as pointed out by Wang and
Zipf (2017), when working with crowdsourced data it is necessary to verify its
reliability, dene guidelines for mappers, and data validation forms.
Among the applications that are most interesting for the purposes of this the-
sis work is “Urban/territorial restoration and seismic risk prevention: a metho-
dology. Learning and experimenting from the case of 2016 Central Italy earth-
quake”. This research project is part of the activity within the ‘Grande Ricerca
di Ateneo della Sapienza Università di Roma (Empler, 2017)’ and is conducted
by the team of the research unit “Urban Seismic Risk: Prevention and Recon-
struction” of the Department of History Design Restoration Architecture of
La Sapienza University of Rome. The focus goes to a specic section of the
project: “ARIM (Assessment Recontrucion Information Modeling) Procedure
Applied to Reconstruction” which proposed the ARIM methodology (Calvano
et al, 2019) developed in the context of the 2016 Central Italy earthquake.
This methodology attempts to manage the complexity of urban aggregates
of minor centers from the urban spatial scale (represented by geodata) to
the architectural scale (through visual survey and instrumental survey). For
the development of the dierent prototypes, the team used VPLs that allow
for the processing and management of heterogeneous data (geometric, hi-
storical, etc.) to be used for modeling and analyzing at the urban scale. The
workow to generate the model can be summarized in two phases: the rst
in which a general three-dimensional model of the urban aggregate is gene-
rated (‘synthetic model’) representing the main volumes and roofs, while in
02.BACKGROUND
94
3DCITYGH: an Expeditious Parametric Approach for Digital Urban Survey
and City Information Modeling of city-block Structural Models
Fig. 51 | Workflow and model-
ling environments of the ARIM
procedure. Source: Calvano et
al., 2019.
the second phase this model is transferred to the BIM environment for further
development, geometric and informative, of the individual building units. In
the latter stage, the product of instrumental surveys such as georeferenced
point clouds obtained through integrated survey methods (terrestrial laser
scanner, aerial drone shots, SFM photogrammetry) is also integrated (g. 51).
95
2.5 Conclusion and research challenges
The common theme that emerges from the state of the art is a major digital in-
novation in the approaches used in the dierent elds (urban survey, parametric
modelling, structural analysis, 3D city models) where the skills required by engi-
neers and architects are pushing towards programming and computer science
knowledge.
Another common point in the dierent elds is the search for greater exibility in
the choice of approach towards the object of the work. There are many attempts
to disengage from black-box processes dictated by software houses in order
to try to create increasingly dynamic and open digital working environments for
interdisciplinary teams.
This leads to another important point, which is the customisation of workows.
This is especially the case in the eld of 3D City Models where, unlike other
workows in other disciplines, the standard formats provide a certain semantic
structure to be respected, but do not impose a certain workow on the way to
achieve it, thus leaving researchers and practitioners to decide on the best way
of working.
The research gaps that arise with regard to the dierent topics discussed are the
following:
1. In the eld of digital surveying, there are works that can speed up acquisi-
tion times, however the technologies used often require a lot of resources
in terms of both equipment and expertise and permits from local institu-
tions (as in the case of airborne digital surveying). The issue of sustainabili-
ty and scalability of urban surveying does not seem to be comprehensively
addressed except for a few low-cost applications related to urban-scale
360 photogrammetry.
2. Concerning parametric modelling, the use of explicit parametric modelling
for urban modelling purposes was adopted mostly for visualisation pur-
poses and/or interoperability with BIM environments. Furthermore, in the
present works, there is no adherence to the semantics required by 3D city
model formats (CityGML/CityJSON). It is also worth mentioning that urban
modelling practices using VPL, in particular Grasshopper, are becoming
more widespread. Nevertheless, the semantic treatment through data tre-
es2 for urban systems is not discussed in depth or comprehensively. In
addition, workows that (semi-)automate urban modelling do not seem to
propose solutions that take into account the use of VPLs despite the fact
that VPLs are already being used in HBIM in monumental architectures.
3. With regards to expeditious seismic analyses at the urban level, the pro-
cesses to apply the mechanistic approach seem to be very time and re-
source consuming. However, the adoption of parametric modelling in the
workow of these analyses seems to be promising in terms of both optimi-
sation and results. It is therefore interesting to expand these applications
to the urban domain by facilitating modelling operations even to very large
and complex objects such as entire blocks.
2 Data Trees are semantic relationships between data inside Grasshopper. These objects define its data
structure.
02.BACKGROUND
96
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