Sensors 2021, 21, 1083. https://doi.org/10.3390/s21041083 www.mdpi.com/journal/sensors
Protocol Development for Point Clouds, Triangulated Meshes
and Parametric Model Acquisition and Integration in an HBIM
Workflow for Change Control and Management
in a UNESCO’s World Heritage Site
Adela Rueda Márquez de la Plata 1,*, Pablo Alejandro Cruz Franco 2, Jesús Cruz Franco 3 and Victor Gibello Bravo 4
1 Department of Graphic Expression in Architecture, University of Extremadura,
10003 Cáceres, Extremadura, Spain
2 Department of Construction, University of Extremadura, 10003 Cáceres, Extremadura, Spain;
3 Panta Rhei Desarrollo S.L., 10003 Cáceres, Extremadura, Spain; firstname.lastname@example.org
4 Arqveocheck S.L., 06810 Mérida, Extremadura, Spain; email@example.com or firstname.lastname@example.org
* Correspondence: email@example.com; Tel.: +34-927257195
Abstract: This article illustrates a data acquisition methodological process based on Structure from
Motion (SfM) processing confronted with terrestrial laser scanner (TLS) and integrated into a His-
toric Building Information Model (HBIM) for architectural Heritage’s management. This process
was developed for the documentation of Cáceres’ Almohad wall bordering areas, a UNESCO World
Heritage Site. The case study’s aim was the analysis, management and control of a large urban area
where the urban growth had absorbed the wall, making it physically inaccessible. The methodology
applied was the combination of: clouds and meshes obtained by SfM; with images acquired from
Unmanned Aerial Vehicle (UAV) and Single Lens Reflex (SLR) and terrestrial photogrammetry; and
finally, with clouds obtained by TLS. The outcome was a smart-high-quality three-dimensional
study model of the inaccessible urban area. The final result was two-fold. On one side, there was a
methodological result, a low cost and accurate smart work procedure to obtain a three-dimensional
parametric HBIM model that integrates models obtained by remote sensing. On the other side, a
patrimonial result involved the discovery of a XII century wall’s section, that had supposedly been
lost, that was hidden among the residential buildings. The article covers the survey campaign car-
ried out by the research team and the techniques applied.
Keywords: modeling; data-fusion; UAV; HBIM; TLS; cultural heritage; cáceres; defensive wall; dig-
The way in which we understand our historic centers today is subject to constant
changes derived from the tools [1–3] we use to interpret them, especially digital tools and
data acquisition and processing procedures. These new digital tools are expanding our
capacities to study the city and increasing our research scope. Not long ago, our studies
were limited to a building’s three-dimensional models, whereas today we can study large
urban areas developing urban scale parametric models. (Figure 1)
The knowledge of today’s urban historical and cultural contexts is determined by the
constant interpretation systems evolution, thanks to the continuous development of dig-
ital tools and data acquisition and processing. At the same time, as mentioned before, the
scale has changed; today we can create urban models that combine different techniques
Citation: de la Plata, A.R.M.; Franco,
P.A.C.; Franco, J.C.; Bravo, V.G.
Protocol Development for Point
Clouds, Triangulated Meshes and
Parametric Model Acquisition and
Integration in an HBIM Workflow
for Change Control and
Management in a UNESCO World
Heritage Site. Sensors 2021, 21, 1083.
Academic Editors: Andrea Giachetti
and Enrico Gobbetti
Received: 5 January 2021
Accepted: 26 January 2021
Published: 4 February 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and insti-
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://crea-
Sensors 2021, 21, 1083 2 of 30
 such as: traditional photogrammetry, Unmanned Aerial Vehicle (UAV) photogramme-
try, terrestrial laser scanning (TLS) and LIDAR models. These models, once combined
through different techniques , can generate a three-dimensional one with different lay-
ers of information. This model allows a highly precise analysis of the urban environment
and, more importantly, a non-invasive tool to document and preserve our cities with an
overall vision . However, these models demand tools and methodologies that simplify
interoperability among them to facilitate the researcher’s work .
Figure 1. (a) Detail of the study area in which the residential growth where the hypothetical military remains can be seen.
(b) Detail of the inaccessible roof area. (c) Detail of the hypothetical intrados urban growth.
In order to develop a methodology that allows us to encode, interpret and finally
manage the historic centers from the complexity of the urban cells, the research presented
in this work develops the study case of a highly reliable and relative easily developed
parametric model of an urban area of UNESCO’s World Heritage City of Cáceres. This
urban area was selected because it is a historic military zone where, until the date of
presentation of this research, the XII century’s Almohad was considered to have been de-
molished to allow the city’s urban growth. The research team had the intuition that hidden
and unknown fragments of the Wall had remained until today.
Thus, this research had a double mission; firstly, to develop a parametric model to
improve our knowledge of the historic centers from an overall view and secondly, to lo-
cate a series of lost fragments of the Almohad Wall that would serve as practical case to
test the validity of the digital methodology presented [8,9].
The works on this area were carried out in 2019. Two research campaigns were car-
ried out. Each campaign had different degrees of intensity: the greater the area studied,
the less detail. The first campaign covered an area of more than 7578 m2 and in the second
Sensors 2021, 21, 1083 3 of 30
campaign the study area was reduced to 1798.94 m2 to allow a more detailed and accurate
These campaigns developed a rapid digital reconstruction of the study areas with
parametric 3d models characterized with different information (Figure 2a). The activities
focused on the definition, testing and development of a methodology based on easily rep-
licable procedures applicable at this work’s scale and at major and minor scales. The work-
flow developed was aimed at optimizing the best use of each instrument: LIDAR, UAV
and reflex cameras (Figure 2b,c), for its integration into a final and replicable model
Figure 2. (a) The key to this project is the use of a Historic Building Information Model (HBIM) tool that serves as an
articulating element of the different databases obtained, guaranteeing their interoperability with a high level of reliability.
(b) Terrestrial laser scanner (TLS) model, processed in Real Work (c) Work in progress of the Structure from Motion (SfM)
model in Agisoft Metashape.
2. Related Works
In the field of heritage and more specifically in the use of BIM (Building Modelling
Information) tools to parameterize reality, there are many researchers who have worked
on workflows that systematize communication between the capture of reality and model-
The incorporation of UAV systems in combination with SfM (Structure from Motion)
methodologies has generated an immense capacity to study our buildings and historical
sites as shown in recent studies [12–14]. What began as study tools for buildings and ar-
chaeological sites [15,16] nowadays has extended to historical urban areas, thanks to the
equipment’s improvements (better and lighter cameras and better computers with more
computing capacity) and to the powerful SfM process software tools in the field of analy-
sis. Today, not only are we capable of analyzing a building but also a city.
Currently, it is necessary to integrate those models obtained by photogrammetry
(ground or aerial) and the point clouds obtained through TLS within the BIM workflow.
Sensors 2021, 21, 1083 4 of 30
This integration and its implementation within real projects with real results is opening a
path of possibilities that we could not imagine a few years ago.
The state of art’s analysis shows us different works of great interest that are currently
being developed and that open a world of vast possibilities when it comes to study our
heritage. Among these ones must be highlighted the advances that are being carried out
in the complex surfaces’ modeling from point clouds using Rhinoceros and the modeling
automation of some elements through Grasshopper . These advances allow us to
model elements with qualities and physical properties (thickness, deterioration, arrows,
turns, etc.) similar to construction ones before industrialization (it is evident that a histor-
ical construction is characterized by lacking orthogonality) The use of tools such as rhi-
noceros allows us to achieve a precision’s high degree when obtaining meshed surfaces
from reality captures. At present, this workflow has also been improved by Rhinocerus
with Rhino Inside REVIT(Rhino-Inside technology allows us to run Rhinoceros like any
other plug-in within Revit and AutoCAD), which opens a range of possibilities that must
be analyzed in depth.
Nevertheless, there exists a traditional modeling that relies on three-dimensional
models obtained by UAV and TLS to recreate buildings. In this modeling we obviously
have to do a processing/study of the information that comes to us in which we synthesize
it, mainly the constructive element’s geometry, to obtain a cleaner and more operational
model . These cleaner construction elements can in turn be exported to other calcula-
tion programs. In short, two different models, one more faithful to reality and the other
more agile or with more interoperability capabilities.
We can extend the scope of the previous model by adding new layers of information.
In this way, as temporal phases, layers of information based (or formed from) on historical
documentation can coexist, such as historical photography, historical drawings or even
descriptions that help us to model disappeared parts [13,19–21].
All the previous models, and without doubts the parametric modeling and the new
ways of understanding the architecture’s drawing, open to us an immense range of pos-
sibilities to improve and facilitate maintenance, conservation and restoration . The
possibilities of incorporating information are practically infinite, as we can incorporate
almost any value that is necessary to enrich our group and walk towards smart cities .
In our case, the proposed workflow was designed to optimize each instrument’s best
use (UAV and reflex cameras), for the integration of terrestrial and aerial photogrammetry
with TLS point clouds in a HBIM (Historical Building Information Modelling) platform; a
platform that optimizes the data’s workflow. On the other hand, it was applied in a real
case. The study case consisted of locating a hidden section of a XII century wall. This sec-
tion was not documented in the historical sources nor cataloged in the current manage-
ment instruments since it had remained hidden among the constructions that had been
attached to it over time.
Previously, the research team had already obtained important results in a nearby area
in which it had located the remains of the Almohad Wall that were considered lost
[8,15,24]. These remains of the Almohad Wall, which were considered missing and were
poorly located in the official planimetry, were located, positioned and preserved through
its inclusion in the city’s Preservation Plan. In that case, the study area was relatively
small, approximately a study area of 500 urban m2, compared to the new study area that
covered about 1798.94 urban m2 (Figure 1a–c).
3. Theoretical Framework: What Are We Looking For?
Architectural elements are subject to a life cycle, thus in the same way that a living
being is born, grows and finally dies; an architectural element has a similar evolution be-
tween its appearance, its development and finally its disappearance. However, sometimes
it happens that this disappearance is not real, but through processes of concealment, what
was considered as eliminated has remained.
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This concealment occurs because the documentary sources do not reflect the exist-
ence of the element, so it is lost in memory; and because the element is not physically
visible or accessible in a direct way, so it is lost in the environment. Finally, the element
falls into the category of the disappeared. This situation obviously avoids its valorization
and preservation since what it does not exist cannot be cataloged (Figure 3).
Figure 3. Comparative diagram of the living being life’s cycle of and a historical building showing the possibility of con-
This case is especially significant in those architectural elements that have been mod-
ifying their function over time, so they have undergone changes to adapt to their new one.
The walls are a paradigmatic case, since over the centuries they have evolved from having
a delimiting and defensive function of the urban perimeter; to being transformed into
dwellings using their walls as parts of these; to being eliminated once they were consid-
ered as obstacles to the development of cities. Furthermore, this evolution has not been
uniform throughout the whole element, and thus different parts of the perimeter can be
found in different stages. Some parts may have already reached a final stage and have
disappeared; others may be present, but have developed a residential use, and others,
however paradoxical it may be, may be emerging, for example, the historicist rehabilita-
tion of a section of a defensive wall (Figure 1a–c).
In the case of the missing walls, these are generally sections that were physically
eliminated by the wall’s demolition processes due to the city’s growth or by its ruin due
to wear or natural disasters; but it can also be a hidden heritage. Concealment may have
occurred because these sections became part of the village mixing with the urban fabric,
and this fact was not documented.
It is in these cases that the application of new technologies can help in a non-invasive
way to make comprehensive models of these complex areas; models that allow us to study
the village from a new perspective in search of hidden remains of the monument; remains
that once located and confirmed by traditional study methods, can be cataloged and thus
preserved and valued.
4. Developed Methodology
4.1. Case Study’s Definition: Northeast Section of the Cáceres’ Wall
The use of a TLS System was justified by the need to generate an agile procedure that
helped us to read the study area’s urban structure and that could be extrapolated to other
city areas. Since these point clouds were referred to an arbitrary and unscaled system, we
used the partial model obtained by means of TLS to register the different models in a
complex system. This system was referred to an absolute coordinate system using control
Sensors 2021, 21, 1083 6 of 30
points that allowed applying the corresponding transformation (scaling, rotation, and
translation in the three axes) . Moreover, this partial model, obtained through TLS,
also helped us to validate the precision of the clouds obtained through SfM.
Once the point clouds’ quality was verified and these were approved, the next step
was to obtain a high-precision topographic digital elevation model (DEM). This model
was made with all the information collected and allowed the research team to discretize
between natural topographic barriers and manmade ones using three-dimensional and
two-dimensional documentation (contour lines) (Figure 4a–c).
Figure 4. (a) Image obtained from the fusion of the 2.5D colored mesh and wire mesh models. (b) Digital elevation model
obtained through a 2.5d procedure from SfM model (c) Sectioned perspective of the polygonal mesh obtained through the
The works consisted of two campaigns as described in the next section. The first cam-
paign (see Figure 4a,b) covered a 7578 m2 urban area. The second campaign worked on a
1798.94 m 2urban area affecting a total of 14 buildings of different types and construction
systems. The second campaign’s area was reduced for operational purposes. In the second
campaign, the study area was densely constructed with few courtyards and empty spaces.
The total area of courtyards was 257.87 m2, 14.33% of the total, as shown in the following
table (Table 1).
Sensors 2021, 21, 1083 7 of 30
Table 1. Second Campaign’s Research area.
Property Floor Area Patio Area Protection Level
1 Obra Pía de Roco 1 211 m2 - Ambiental singular + integral
2 Obra Pía de Roco 3 67 m2 - Ambiente singular
3 Obra Pía de Roco 5 56 m2 - Ambiental singular
4 Obra Pía de Roco 7 262 m2 19.63 m2 Ambiental singular + ambiental
7 Adarve del Cristo 1 556 m2 172.36. m2 Ambiental + VPS private garden singular
8 Calle Hornillos 2 157 m2 13 m2 Generic
9 Calle Hornillos 4 92 m2 16.48 m2 Ambiental
10 Calle Hornillos 6 80 m2 8.8 m2 Ambiental
11 Calle Hornillos 8 153 m2 27.6 m2 Ambiental
12 Calle Hornillos 10 - - Ambiental
13 Calle Hornillos 12 85.62 m2 - Ambiental
14 Calle Hornillos 14 79.32 m2 - Ambiental
This area consisted of a heterogeneous set of residential buildings asymmetrically
arranged. The set was limited by three streets: Obra Pía de Roco Street, Adarve del Cristo
Street and Hornillos Street. Regarding the interior the study area, we started from the
premise that there were interior patios that, depending on their location and disposition,
could offer us some information about the lost wall. However, we did not have any topo-
graphic plans that showed what happened inside the area such as the buildings’ sections,
how these rested on the ground or the streets’ configuration.
Cacere’s old city residential construction is characterized by the reuse of previous
constructions to save means and economize . This fact has created a very interesting
residential architecture with vaulted houses that combine different structures from differ-
ent times  including the military structures among them the XII century Almohad Wall
or the 19th century adobe walls. The SfM acquisition was aimed to optimize the move-
ment of the DSLR (digital single lens reflex) camera to generate chunks of high-density
points and integrity of each surface [27–29].
In order to collect roof data and obtain roof information, in some cases it is essential
to use UAVs that allow us to carry out a morphological analysis of the urban landscape,
obtaining another second SfM model on a smaller scale [30,31].
It is necessary, as we have already said, to integrate the SfM models obtained both
by SLR and UAV cameras with a TLS (terrestrial laser scanner) model to be able to verify
the confidence in our point cloud, assess deformations, scale losses. It should be noted
that in many areas we will access areas where TLS systems have not been able to access,
which is why they are complementary systems.
Finally, in an HBIM model, we will have a platform that combines different paramet-
ric models, serving as a hub to work with point clouds/obtained through TLS, SfM,
meshes, textures, etc.)  and thus have reached our management and control tool 
(See Figure 5).
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Figure 5. Workflow 1: Cabinet and field works for the first generation of 3D models using SfM and TLS prior to incorpo-
ration into HBIM. First results through presentations.
Sensors 2021, 21, 1083 9 of 30
4.2. Planning Works: 1st Campaign TLS Acquisition and 2nd Campaign Aerial and Terrestrial
4.2.1. Campaign: TLS System
The objective of the first campaign (see Figure 6a,b) was to generate a first closed
polygonal model of the study area based on a high-precision point cloud taken with a
focus X130 laser scanner. In this campaign, 107 scans were performed. The entire set of
scans added up to a total of approximately 182 million points.
The polygonal survey consisted of scans approximately 30 m apart. This procedure
attempted to achieve two objectives. The first one was to obtain information about the
ground and the vertical elements, which in some cases were house facades and in other
were wall elements or defensive towers. The second one was to mark the location of the
targets using geopositioned steel tips to allow us to retake and expand these scans in the
future and reference them into the main model; so, if in the future we had the possibility
of accessing a property, we could go back to the physical mesh of steel spikes placed in
the pavement to continue expanding our model obtained through TLS [27,33].
Figure 6. (a) Blue: area of action compared to the intramural area of Cáceres. Orange: remains of
the Almohad Wall (b) Detail of the area of action in blue and of the supposed conserved remains
known before the results of the present investigation
The procedure followed within the research project  for data collection was based
on a high-density sphere mesh separated by a maximum of 30 m (see Figure 7a,b).
Sensors 2021, 21, 1083 10 of 30
Figure 7. (a) Plan of the mesh established for the placement of the spheres for the TLS (b) Detail of
target concentration areas to ensure their visibility in subsequent campaigns. (c) Placement of one
of the spheres using a topographic nail.
The position of each sphere within the mesh was geolocated on the pavement using
easily locatable topographic marking nails. All these nails were clearly and concisely po-
sitioned in a GPS (Global Positioning System) with centimeter precision locating each el-
ement in absolute UTM (Universal Transverse Mercator) coordinates (see Figure 7c).
This high-density mesh was essential for the research project as we understood the
models under elaboration as living models. Regarding living models, on one hand, be-
cause the city is alive and grows responding to new needs, we may need to expand them
to reflect these changes. On the other hand, because the models were not complete, in
future studies they would have to be expanded with the interior of the surrounding
houses, as at present the studies had only looked for the set’s geometry.
This dense mesh served as an anchor point between the three-dimensional models.
As mentioned, all the typographic mesh points (see Figure 7a,b) were geolocated and
shared by all the models, using anchor points to interconnect all the three-dimensional
This first model obtained was incomplete as it did not have the capacity to obtain
data relating to roofs, courtyards, and interior facades. This model allowed us to screen
the study area, reflecting the topography and the height of the buildings. Through this
screen we were able to geolocate the wall’s intrados and extrados to know the different
heights of the barbican, this was not possible until this moment as this urban area was
occupied with residential buildings that avoided a clear view of the area.
Once this area was analyzed (Figure 6a,b), we proceed to define what it would be the
2nd campaign carried out by means of a UAV team and an SLR (Single Lens Reflex) team
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Figure 8. (a) 2nd campaign study area, each quadrant divides the 7 surveying missions carried
out. Orange: known remains of the XII century wall; blue: 19th century residential growth that
have absorved military constructions. (b) Fused model of the triangulated mesh and the textured
mesh in which the methodology followed in the different missions when carrying out the flights
can be appreciated.
4.2.2. Campaign: Aerial and Terrestrial Image Acquisition
The great distance between the two streets that limited the study area, the morpho-
logical diversity of the residential units that composed it, and the impossibility of access-
ing certain areas (unoccupied houses, roofs, etc.) led to the inevitable choice of an acqui-
sition program Image by UAV complementary to images acquired by terrestrial photog-
This campaign took place in the first half of 2020 (months of January and February)
with a small Dji Mavic Mini team weighing less than 250 g, as a non-profit or commercial
informative activity and outside the birds’ breeding period (April-May to September-Oc-
tober). As it was a flight with a leisure drone without a combustion engine and the area
Sensors 2021, 21, 1083 12 of 30
was part of the Natura 2000 European Ecological Network, we complied with Decree
110/2015, of May 19th, in its Annex I, and that did not require an affection report.
This Dji Mavic Mini was the UAV used to create the point cloud of the entire study
area delimited in the second campaign. For this, this second study area had been divided
into 8 study areas, each corresponding to one flight mission of the UAV team, which was
limited by flight time, weather conditions, operator visibility.
These flights were integrated with terrestrial photogrammetry with an SLR camera
with the objective to generate a parametric HBIM model that combined the different mod-
els: the one taken with UAV and SLR and the one taken with TLS in order to assess the
quality of the different point clouds and their geometric reliability [14,27,35].
4.3. UAVs and Terrestrial Acquisition and Post-Processing: Scan to BIM
The use of photogrammetry and laser scanners to study Heritage is especially inter-
esting as it allows capturing highly complex buildings with a very high degree of preci-
sion without the traditional data collection’s limitations . The integration of these tech-
niques in the HBIM workflow is a step forward to guarantee our heritage’s preservation
and maintenance and to design interventions on our heritage more respectful with the
The validation and checking model were obtained using TLS Faro Focus, processing
the point cloud using the Faro Scene software [24,25]. With this software, an alignment of
the different scans was carried out semi-automatically thanks to the spheres described
above. We decided to organize the point cloud not based on the scans but based on the
construction elements that interested us for the interpretation of the historical set. Accord-
ingly, since we were conducting a non-invasive analysis of the wall, we made groups of
residential buildings’ roofs and facades, Roman archaeological remains, Almohad archae-
ological remains, and reconstructions, as shown in Figure 9a.
Figure 9. (a) Organization of the TLS model according to scan locations that match building elements in RECAP. (b) Visi-
bility chart of the different graphics in which we can activate or deactivate the constructive elements as they are organized
according to the scans.
The minimum precision values to validate the TLS model and that this later serve us
to check the photogrammetric models were between 2 mm and 4.00 mm. This level of
precision was not guaranteed either on the roofs or in the areas close to them where we
had much larger deviations. All artifacts were removed from the model to improve its
understanding. The project, as already indicated above, had some anchor points that had
been georeferenced and to help us to link the different models. These control points were
those described above and that made up the work mesh. In this point cloud, a resample
of the point cloud was carried out prior to export. Once this TLS model was validated, it
was exported in * LAZ format (file format containing point clouds) and imported into
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RECAP(intellingent software for 3d modelling) to prepare the cloud for import into
REVIT (Figure 9b).
The photogrammetric UAV model was developed with the Agisoft Metshape 1.6.2
software to generate a dense point cloud for each of the seven missions. Each of the mis-
sions was treated as an independent work block  for which sparse cloud, dense cloud,
mesh, and a digital model of DEM elevations were obtained. Subsequently, the models of
the seven missions were aligned, obtaining a general model composed of 1744 cameras
that resulted in a dense point cloud of 75,676,246 points and 6,135,248 polygonal faces.
The point clouds obtained in each of the missions were aligned with homologous points
identified in the streets. Note the versatility of this new tool as it allows us to reduce noise
and artifacts from the models.
From this photogrammetric processing we obtained two data outputs to incorporate
into our parametric model. The first one was a point cloud whose preparation process to
insert it in REVIT was similar to the procedure followed with the TLS model, but in this
case, Agisoft provided us the tools necessary to classify the points, clean the model, and
resample it. Before exporting this model to REVIT, it was necessary to make a previous
step through RECAP in the same way as we did in the TLS model.
The second data output was a three-dimensional mesh model with textures. This
model in * DAE, * OBJ or * 3DS format was incorporated into our parametric model (note
that in the case of REVIT, unless we create families with Formit software, also from Auto-
desk, they will not be imported). To incorporate this model in REVIT, a previous step was
carried out in the free distribution Blender software to allow us to export an * ifc file (note
that although derived from the tests carried out, this step could also be carried out in the
Sketch up software). Once we made that export, we incorporated it into our parametric
model (note that in the case of wanting to use the obtained textures to elaborate photore-
alistic images we will use the * DAE files, in our case, we used Lumion, but we could also
have used Rhinoceros that requires installing different rendering engines like Enscape).
The photogrammetric model from the terrestrial data collection was elaborated with
photographs in raw format to maintain the highest quality and integrity in the starting
documentation. This documentation was processed in Agisoft Metashape obtaining the
same three-dimensional models obtained in aerial photogrammetry with the limitation
that there are large blank areas (ghost zones) and the distortions in the data collection
when the photographs had not been taken orthogonally . The missions carried out to
obtain the terrestrial photogrammetric model did not overlap with the missions carried
out to obtain the aerial model. The same methodology was followed as with the TLS sys-
tem, but in this case overlapping both campaigns in order to have the same reference
points and thus obtain time .
Note that taking data in RAW (it is a format to store images without compression)
format instead of jpg format (see Table 2) in both aerial and terrestrial photogrammetry is
recommended [36–38]. We will have a considerable increase in the size of the files when
obtaining them from the raw camera but during the processing we will be able to improve
different aspects of the photography that will improve our results especially in low light
situations. Among the modifiable parameters will be the following: white balance, expo-
sure, contrast, highlights and shadows, intensity, saturation and focus. All these values
will allow us to improve our data collection in the cabinet if we have lighting problems,
improving our results.
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Table 2. Comparison table between jpg format and raw format RAW.
1 Raw 16 bit/14
bit/12 bit *
Higher image quality making the most of the camera’s capture and to-
It is its own format, which requires the use of specific programs or
plugins depending on the camera used.
2 JPG 8 bit Yes Yes
The image is compressed and reduces the image quality. The tonal
range of our camera is reduced.
It is a standard format recognized by most devices and programs. It is
easy to share.
* It depends on the camera chosen and the level of compression. Each brand has its own raw format and its variations
(Canon CR2, Nikon NEF, Sony SR2, Panasonic RAW2, Olympus ORF, Pentax PTX, etc.). ** To calculate the percentage
increase in size, a Canon EOS 400D camera with 18 MP Megapixel CMOS has been taken as reference. *** The percentage
has been calculated with respect to average values (25 Mbytes for RAW and 8 Mbytes for JPG).
Finally, another meshed model was obtained, especially interesting for our study, a
digital elevation model. This model allowed us to establish the altimetric elevation of the
existing interior courtyards in the residential units and as a result allowed us to develop
a theory of the possible primitive topography of the wall and its barbican in a non-invasive
way using a 2.5D triangulation.
4.4. Validation Process of the Different Outputs: Determine Reliability
One of the key points in this process was the geometric validation of the different
point clouds and models. The reality is that for the proposed work, data collection by UAS
(Unmanned Aerial System) is transformed into an agile and versatile system that guaran-
tees a very complete model by lacking ghost zones due to its ability to reach almost all
points. However, the reality is that the three-dimensional model obtained through soft-
ware processing may contain geometric inaccuracies that must be checked, which is why
this high-precision model obtained through TLS has been proposed, since we know its
millimeter precision, which can validate our other models, serving as control and valida-
tion of results [5,27].
The process consisted of four steps, in essence, each of the systems used (TLS, terres-
trial photogrammetry, and aerial photogrammetry) provided us with its own three-di-
mensional model with its strengths and weaknesses. The TLS system guaranteed us a
point cloud of centimeter quality and an agile tool to expand the three-dimensional model
in future missions as long as we kept our anchor points on the study area’s ground. The
photogrammetric models were characterized by being a low-cost system that needed a
validation process to guarantee that the data collection was correct. The terrestrial photo-
grammetry was more precise but had more ghost zones. Finally, the aerial photogramme-
try was less precise in principle, but reduced the number of ghost zones to almost 0.
As mentioned, in this procedure we obtained the different clouds to scale and orient
them in our case in the RECAP (Figures 10 and 11) platform to facilitate and guarantee the
passage to a BIM (building information modeling), in our case in REVIT platform, as they
both belonged to the set of Autodesk tools where interoperability was guaranteed (Figure
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Figure 10. Area view of the two-point clouds to determine the reliability of the models (UAV model and TLS model).
Figure 11. (a) Perspective of the fused model in which we can see the limitation of data collection with the TLS system as
its vision is limited (blue: TLS model overlap with the UAV model (purple)) (b) One of the check sections in which the
reliability of the point cloud taken by aerial photogrammetry can be estimated by comparing it with the data collection
using TLS, the precision of which is known (centimeter). Blue: TLS model overlap with the UAV model (purple).
Note that when we have the point cloud in other formats, such as psx from Agisoft
Metashape, an intermediate step is necessary, which consists of transforming that cloud
into a format suitable for Recap. For this, we export from psx to laz. The laz format allows
us to load that cloud in Recap, where we can later link it to REVIT. Step 2: when we are in
REVIT platform, if we already have the cloud in Recap format, we can link it directly in
REVIT as a point cloud that we will call 1 A. We can have as many as we want, 1 A, 1 B, 1
C, etc. The union of the point clouds will be carried out by means of homologous points.
Sensors 2021, 21, 1083 16 of 30
Once all the clouds have been fitted in the same position, we proceed to a chromatic dif-
ferentiation to easily visualize the geometric differences. To do this, we assign each cloud
a color, and through volume cuts, 2D sections, plants, we check the different geometries.
From REVIT, the handling and superposition of the different point clouds is tremendously
agile, being able to activate and deactivate zones to improve the reading of the set and
obtain comparative sections of the models in which to check our level of precision .
In the previous text we have not referred to the mesh models described above be-
cause these are the result of the processing of their corresponding clouds. If the point
clouds are validated by extension the mesh models will also be validated. It is not neces-
sary to perform this process on them too. All the steps of the process are summarized in
Figure 12. Validation process of the different geometries obtained in four steps.
Sensors 2021, 21, 1083 17 of 30
5. Unified HBIM Model
Once we validated the geometry of the point cloud, we performed the step reflected
in Figure 13. In this step, we had two workflows, each of these flows had a different ap-
proach described in Table 3.
Table 3. Workflow after geometric validation.
Management Control Conservation Planning of Interventions
Workflow 1 x x x x x
Workflow 2 x x X
A first “work flow 1”: survey, control and project and a second “work flow 2”: control
and virtual restitution. These two flows guarantee the maintenance, control, conservation,
planning interventions, project and dissemination of our heritage based on parametric
bases that can be updated with new information.
The first workflow is developed entirely within the REVIT platform (it is the contin-
uation of the geometric validation carried out in Figure 12) and the result is a central co-
ordination model that combines the obtained models that are described in Table 4. All
these models, as indicated in Figure 13, are coordinated in a central model in the REVIT
platform, linking them as independent files that can be modified and updated in case the
study is expanded to new areas.
Table 4. Three-dimensional models incorporated into each parametric model.
Point cloud from TLS X
Point cloud from SfM from UAV X
Point cloud from SfM from SLR x
3d Mesh with out textures X x
3d Mesh with texture x
3d objects x x
3d MEP (mechanical electrical and plumbing) objects x
The models need to be integrated within a model as if they were disciplines and allow
advanced modeling of the whole. Among the advantages of the REVIT platform is its agil-
ity for manipulating and moving the overlapping three-dimensional files, being able to
use different views depending on the needs. Thus, we can work at all times as if we were
in a traditional CAD (Computer Aided Design) program, activating and deactivating the
layers according to our needs .
This model allows us to perform non-invasive control tasks  by integrating the
different results of our works (TLS model, SfM photogrammetric models and SfM terres-
trial models); all these validated results with a centimeter precision. This workflow allows
the BIM administrator/BIM architect to control and inspect the monuments’ geometric
and volumetric dimensions, practically without effort guaranteeing the building’s control
and maintenance in a completely efficient way.
In addition, these models work with an on and off layer system (see Figure 9a,b), a
useful tool for studying buildings and their surroundings. They allow us to eliminate non-
invasive city’s overlapping layers that make difficult to read the monument chronologi-
cally, but if sequenced, they provide invaluable information about our heritage, allowing
non-invasive readings not done till now. These readings are necessary for the study but
also for the dissemination because with the new virtual reality technologies we can make
visits of almost any type (inspection, study, dissemination, etc.)..
Sensors 2021, 21, 1083 18 of 30
Figure 13. Workflow in three steps and two tours depending on the needs: survey and control and project and virtual re-
The approach adopted for the modeling was to establish a workspace in which we
could visualize the wall’s remains undoubtedly preserved. This way, modeling from a
distant scale to a close one [18,39,40], the military elements that were preserved in the
cloud points were prioritized and discretized to allow making hypothesis about possible
layouts. Approaching us from a far scale to a close one is an orderly methodology that
helps us make decisions and in study these urban groups or urban aggregates . It is
important to follow a methodology of this type because in making decisions we will only
focus on one aspect of the monument; in the first instance we can analyze the trace to later
see the thicknesses or changes in thickness; see Figure 14a–c.
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(a) (b) (c)
Figure 14. (a) TLS model in REVIT with gosht zones (b) TLS model + 3d Mesh without textures completing gosht zones
(c) Virtual reconstruction of the wall.
In a historical building it is common both that there is no orthogonality and that there
are considerable changes in thickness in the same element. Both features hamper the
REVIT workflow. In our model and given that our objective was to know the true geom-
etry of the wall, we ignored the issue of orthogonality, obviously the position of an ele-
ment is what it is and that is a priority. On the other hand, in the modeling of the missing
elements and the assumptions, we did not prioritize the thickness, having a tolerance
range in which we admitted deviations of +/− 5%; see Figure 14c .
This model is a clearly versatile model on which you take measurements. On the
other hand, being parametric, the incorporation of information is always guaranteed, be-
ing able to have a living document with many levels of information, from archaeological
layers, materials, finishes, sanitation networks, plumbing, etc.; see Figure 15a, b and c.
Note that in this second workflow we will prepare the models to be used either in
augmented reality or in virtual reality. We highlight the importance in this case of the
mesh models with texture, as indicated in Table 4, because they will guarantee this pho-
torealistic experience [42–44].
It is important in this workflow to adapt the number of polygons of our three-dimen-
sional meshes because excessive numbers of polygons will make it impossible to use vir-
tual reality tools. This reduction process in our case has been carried out directly from the
Agisoft tool in the same way that we will resample the point clouds obtained by photo-
grammetry. There are other tools on the market that allow us to carry out these operations,
such as free distribution blender or Rhinoceros . The final objective is to obtain a mesh
with few triangles but with a texture with high resolution.
Once we optimized our three-dimensional model, we used the Enscape rendering
engine installed as a plugin in the Sketch up software. The sketchfab.com platform was
used to generate models posted on the network to view our work both as a traditional
three-dimensional model, and as augmented reality and virtual reality. Note that in
REVIT and in Rhinoceros, we can generate virtual reality spaces from our work.
Both these three-dimensional tours in augmented reality and virtual reality as tradi-
tional renders in engines such as Vray, Endscape or the Lumion software allow us a hith-
erto unknown disclosure of heritage; Figure 15 a-c
Currently, it can be combined with calculation programs by having an IFC-type ex-
change file to guarantee interoperability between the different platforms, we can create
measurements and join them to our file, and we can even work with video and rendering
Sensors 2021, 21, 1083 20 of 30
Figure 15. (a) Example of a stratigraphic map superimposed on a parametric model on a wall tower (b) On the left, para-
metric model merged with all layers activated. On the right, non-invasive inspection of the interior walls once the external
layers have been deactivated.
The results of this work are compatible with data management advanced techniques
and technologies. It is important to keep in mind the ability to provide personalized useful
data to each user. In this sense, the conclusions of the project combined with these new
technologies such as the KYRA program (Knowledge-based Information Retrieval from
Art collections) can be very useful to present in a more friendly and directed way what
was discovered in the project .
This type of technology also admits new dissemination channels such as App, Web
content, etc., that help to promote knowledge and dissemination of Cultural Heritage. It
enables dissemination in a format that allows researchers to improve knowledge without
having to diminish scientific or research quality.
An example is a multimedia guide that based on the results of this project that has
been presented to the scientific community, as part of the KYRA program. This example
demonstrates the parameterization’s versatility of this research project data.
Sensors 2021, 21, 1083 21 of 30
6. Discussion and Results
6.1. Discussion and Results of the Data Collection
From the point of view of the methodology the results for data collection revealed
that in a scenario in which we need a low-cost system to capture a constructed reality, we
could utilize a data collection system based solely on aerial UAV systems, defining control
points to validate the cloud point’s quality obtained. A high number of geometric valida-
tion points could even allow us to avoid using TLS systems, thus reducing the campaigns’
data collection costs, although if the economic means and the importance of the study area
allow it, the use of both systems is highly recommended (Figures 16 and 17).
Figure 16. Results for dissemination and study: merged model emphasizing the different stages.
Note how the mesh obtained by SfM eliminates ghost areas.
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Figure 17. (a) Integral model playing with different textures and opacities of the materials to improve the understanding
of the layers. (b) Virtualization modeled from the different three-dimensional models (c) Artistic vision of the whole merg-
ing the different layers with different degrees of opacity to improve the understanding of the whole.
On the contrary, the incorporation of UAV systems in combination with SfM meth-
odologies and the substantial improvement in the quality of cameras, lenses and stabili-
zation systems made the three-dimensional model obtained by acquiring ground images
practically unnecessary. In the SLR model we can obtain high-quality results thanks to the
high-resolution cameras that we have at our disposal as researchers, although we will
always be limited to work at ground level, producing three-dimensional models with
large phantom areas (roofs, interior facades, etc.). Also, data collection through UAV sys-
tems will always be limited by the sectoral regulations that may limit flights depending
on the cases.
The proposed workflows (Table 3 and Figure 13) are an important step in shaping
historic SMART cities [42–45]. Synthetically both workflows cannot be understood sepa-
rately, both allow us to protect our Heritage in its broadest dimension. As we have seen
from maintenance, to control, rehabilitation, and enlargement, etc., new measurement
techniques, especially photogrammetric methods compared to TLS technology, allow
rapid acquisition of reliable data. The article presents how to use different photogramme-
try techniques and the verification of the quality of the information through TLS shots
(Figures 10 and 11).
The conversion and import procedure of the different data bases it still requires sev-
eral software packages for the processing. In our work we have incorporated the different
databases in the parametric model elaborated in REVIT, implementing all the different
data sets, points clouds, three-dimensional meshes, textured meshes (Figures 9 and 14).
Sensors 2021, 21, 1083 23 of 30
This HBIM model allows us to see in real time the different models (mesh, points,
construction phases, elements demolished or to be demolished) creating a powerful asset
management tool that can serve all stakeholders [34,47] (Table 4).
Once the different meshes have been superimposed, we have found an efficient and
fast tool to be able to analyze our Heritage.
On the other hand, with these low-cost shots, we have a very interesting tool to dis-
seminate our heritage. It is no longer necessary to be on the site to see a monument. The
new techniques of total immersion such as VR (Virtual Reality) or AR (Augmented Real-
ity) allow us to democratize knowledge by making t3d models available to anyone with
an internet connection. From simple models that allow us to know the building to the
incorporation of other information such as texts, planimetry, current or historical photo-
graphs to more complex models with augmented reality or even total immersions thanks
to virtual reality and the use of more expensive equipment such as Oculus Rift or HTC
lives (both virtual are virtual reality glasses).
6.2. Discussion and Results of the Analysis of the Case Study
From the point of view of our case study:
Firstly, hidden geometries were revealed showing an unknown urban layout not
documented in the existing planimetry. This geometric evidence showed architectural re-
mains prior to the residential development. These remains, due to its geometry and di-
mensions, corresponded to a military architecture, probably the old wall that supposedly
had been demolished.
Secondly, once our 2.5 D model was elaborated, we encountered two situations in
the courtyards belonging to the dwellings studied. The first one was that the courtyards
were aligned with that hidden geometry that we had located in the previous point (see
Figure 18). The second one was that the patios marked a topographic unevenness of di-
mensions that would only make sense if they had been executed for military purposes.
Note that there was no realistic planimetry of the study area to know the different eleva-
tions in the area prior to the study. (Figure 18).
Figure 18. Altimetric analysis of the study area from the 2.5D model.
Sensors 2021, 21, 1083 24 of 30
Therefore, contrary to what it was reflected in the City Master Plan the Almohad wall
had not been demolished, as shown in Figure 18. The wall still existed; however, it was
not located where it was assumed as it was displaced between 5 and 6 m. The wall, instead
of being demolished during the residential expansion, was used as a foundation and fac-
ing of the new dwellings to save costs and materials.
To confirm the validity of the methodology and check the fact exposed, the research
team studied the historical cartographic documentation to check where the wall was in
this area in the past (Figure 19). For this, the cartographic resources published by the Cá-
ceres’ City Council were consulted. Surprisingly in three of the documents analyzed, in
the study area the Almohad Wall was in the location determined by our methodology and
not where the current plans mark it. This location was maintained until the end of the 19th
century (the military zone appears graphically for the last time in the water supply plan
of 1895 and previously in two other documents of 1813 and 1853 respectively), being dis-
placed in the later representations.
(a) (b) (c)
Figure 19. Cartographic analysis. (a) Column 1 Baibier’s Plan of 1813 and detail of the study area of (b) Column 2 Francisco
Coello’s Plan of 1853 and detail of the study area of. (c) Water Canalization Project of 1895 and detail.
In the first document, Baibier’s Plan drawn in 1813, the wall still existed in this area
and its intrados had not yet been occupied by residential dwellings. Also, the two wall
sections, the one ending in Obra Pio de Roco and the one starting at Arco del Rey were
not aligned, perhaps to create an elbow access.
In the second document, Don Francisco Coello’s Plan drawn in 1853, the wall’s sec-
tion under study appeared displaced several meters in relation to the actual plans, exactly
where this document represents it. Indeed, residential dwellings already appeared in this
Sensors 2021, 21, 1083 25 of 30
Plan and instead of being placed on the extrados of the canvas, they used their intrados,
which makes a lot of sense because obviously towards the extrados we would have a slope
that would make it impossible to use them.
The last document analyzed was The Drinking Water Supply Plan drawn in 1895. In
this document, the wall was represented in the middle of the residential dwellings be-
cause, in the previous case, the wall was being used by these as part of their facings.
7.1. Conclusions Regarding the Proposed Workflow
The article analyzes a workflow for the generation of three-dimensional models from
TLS and aerial and terrestrial photogrammetry, applied in a concrete case of the urban
context of the City of Cáceres, a World Heritage Site. The lessons learned and conclusions
of this workflow divided into two parts: field work-data collection, and office (obtaining
three-dimensional models from the different data collection and validation). Thus, we
have the following:
The preliminary analysis of the urban configuration is an operation of paramount
importance to program the work missions and thus optimize the data acquisition cam-
The use of UAVs as a tool for data acquisition becomes a tremendously useful tool
that enables us to obtain three-dimensional models unthinkable with traditional tools
such as terrestrial photogrammetry and TLS. The use of UAVs guarantees the collection
of data from large areas of land quickly and efficiently.
The use of TLS systems to calibrate the three-dimensional models obtained by aerial
photogrammetry confirms the reliability of these models. The fusion of both models in a
HBIM parametric environment allows us to generate a complex model with different lay-
ers of information that guarantee a surprising quality of information.
The models obtained by terrestrial photogrammetry become less effective outdoors,
perhaps being relegated to future extensions of the parametric model in which interiors
of buildings or areas of poor accessibility and maneuverability are incorporated in which
a terrestrial system is operational.
As mentioned, the data acquisition, post-production and verification of the quality
of the three-dimensional models obtained by UAV was tested in a particularly complex
part of the city, characterized by a dwellings’ agglomeration with different volumes that
hided the geometries of previous elements, such as the Almohad Wall. There was a certain
uncertainty in the quality of the final database that we could not leave behind, so we had
to carry out constant checks that guarantee the quality of the information. For geometric
validation, it was essential to have the equipment and work procedures that guaranteed
centimeter precision. In our case, the use of the point cloud taken through TLS and the
use of the georeferenced mesh with a centimeter GPS allowed the creation of a database
suitable for the validation of the three-dimensional models obtained by photogrammetry.
In the case of not having TLS equipment, the use of topographic instruments, such as total
stations, becomes essential to control the error of the different point clouds. In our case,
the verification with the TLS model gave satisfactory results.
The models’ fusion in a final intelligent digital platform that allows us to interact
with the different models and serves as the basis for both the control and maintenance of
the Heritage and opens research future lines for future projects is the study’s final result.
In this new digital era, parametric models in BIM technology allow us to link what was
previously unthinkable: a multitude of point clouds, three-dimensional meshes, textured
models, etc. This in turn allow us to generate a parametric model in continuous change
and evolution; with the capacity of being updated as our society or our ability to incorpo-
rate information advances; and of working three-dimensionally with all this information
in real time, allowing us to obtain cuts, sections, elevations, topography, etc. On the other
Sensors 2021, 21, 1083 26 of 30
hand, the archaeological layers have also been incorporated on the three-dimensional
models, obtaining a model with the incorporated stratigraphy .
We remark the possibility of incorporating augmented reality and virtual reality
technologies thanks to workflow 2. Making our monuments available to the population
through these technologies is essential to democratize architecture by facilitating access
to sites that until moment were inaccessible . This universalization will guarantee that
we all know our past regardless of where we live and the distances, thus access to these
databases from schools will allow any child to access a monument from their school with
From an educational point of view, these databases and this new way of interacting
will take us a step further on what we know, in the same way that in the 1980s we used
paper encyclopedias, in the year 2000 tools such as digital photography appeared, and
videos (all of them in 2d) were driven by the democratization of Internet knowledge. In
the year 2020, favored by technology, we will access cultural sites through a computer.
From a social point of view, these databases will guarantee a universalization of our
heritage through new technologies (Figure 20). These first steps in that universalization
will generate the possibility of visiting a bell tower or a church not limited by our physical
condition or distance. Thus, people with reduced mobility will be able to see what has
only been for generations allowed to people without limitations, such as a bell tower.
Figure 20. Virtual recreation of the localized remains superimposed on the three-dimensional model.
7.2. Conclusions Regarding the Findings
First, the wall’s section under study is a genuine fragment of the Almohad Wall that
had supposedly disappeared [16,17]. It is a powerful canvas, which according to its di-
mensions, layout, and location with respect to the orography of the land, is undoubtedly
part of the Almohad Wall that defended the City of Cáceres. This section is built with
masonry. This masonry may be the substitute for the very degraded adobe concrete walls
on this flank, the one most exposed to the attacks of the northern armies in the various
sieges that suffered the city during the XII and XIII centuries. In any case, the section can
only be seen from the courtyards of the houses as it is integrated into the homes; therefore,
Sensors 2021, 21, 1083 27 of 30
an archaeological study in detail is necessary to determine the different construction pe-
riods through a stratigraphic study.
Second, the location and position of the wall’s section under study corresponds with
what is reflected in the historical cartography of the city. In this planimetry it is reflected
that the section in question never disappeared, as it was reused and integrated in the con-
struction of the houses. However, in the current city’s cartography, the wall’s location in
this area is misrepresented (Figures 19 and 20).
In the current recreation plans of Caceres’ Almohad Wall, the wall’s section under
study is aligned with the front facades of Calle Obra Pía de Roco, whereas this study
demonstrates that it is aligned with the rear facades of these houses. This displacement
was already demonstrated in Calle Adarve del Cristo after the rehabilitation of house No.
5 and continues in Calle Obra Pía de Roco as it follows the orography of the area. Further-
more, this displacement corresponds to the location of the wall in the historical plans.
The study of the city’s cartography and documentation leads us to other interesting
findings regarding the doors and shutters of the wall in this area. A shutter located on
Calle Adarve del Cristo n° 5, named San Miguel’s shutter, and a gate, named Arco de
Caleros, reflected in the Francisco Coello’s Plan of 1853; this gate would explain the width
of Calle Caleros at the t numbers 42 and 44. These findings are being studied by this re-
search group to complete the research of the wall in this area (Figures 20 and 21).
Figure 21. Visual inspection of the discovered walls. (a) View of the patios and how the new
houses are attached to the pre-existing wall. (b) Detail of the old mechinales used to build the wall
(c) Imposing arch and barrel vault under one of the houses (d) Start and detail of an existing arch.
Sensors 2021, 21, 1083 28 of 30
Author Contributions: P.A.C.F. conceived and designed the research. A.R.M.d.l.P. has carried out
the research and analysis of the results. P.A.C.F. conceived and carried out the research about sen-
soring and the methodology. J.C.F. has been charge of the management of the research and the
team. V.G.B. has been in charge of the archaeological study and its three-dimensional interpreta-
tion. All the main text has been written by all authors equally. Software, P.A.C.F.; Supervision,
J.C.F.; Visualization, A.R.M.d.l.P. All authors have read and agreed to the published version of the
Funding: This research received no external funding.
Acknowledgments: The broadcasting of this work has been possible thanks to: The funding
granted by the European Regional Development Fund (ERDF / FEDER) and by the Junta de Extre-
madura to the research group COMPHAS through the aids with reference GR18032; This research
has been developed thanks to the TAD3 Lab work program, Techniques in Architecture [Drawing
& Digitalization & Documentation]; This publication has been partially developed thanks to the
SEXPE Innovation and Talent Program of the Junta of Extremadura; Jaime Ruiz Sánchez for his
collaboration in outdoor and office work; Antonio Cuenca Riesco for his collaboration in outdoor
and office work; Consorcio Ciudad Histórica de Cáceres; Juan Manuel Honrado, Rumualdo Gra-
jera and Fátima Gibello for their generosity and attention for data collection from their homes;
Pitarch family, Miguel Ángel and Isabel, for their willingness and generosity in collaborating on
this project; Association of Neighbors of the Monumental Zone for their interest and support for
the research topic; Instituto de Investigación en Patrimonio (i-PAT).
Conflicts of Interest: The authors declare no conflict of interest.
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