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Employing Volunteered Geographic Information in Space Syntax Analysis


Abstract and Figures

The application of volunteered geographic information has rapidly increased over the past years. OpenStreetMap (OSM) forms in this context one of the most ambitious and promising projects, providing consistent global coverage of street network information. With a constantly growing number of participants and the implementation of governmental and proprietary based information is a complete coverage of global street networks within reach. The data allows comparative cross-country analyses and any method developed within its framework are transferable to other cases. This makes OSM a powerful and desirable data source for applied network analyses, such as space syntax. However, OSM data does not come without obstacles. Inconsistent representation of space, topological fragmentation and accuracy are just some of the problems that one faces when employing OSM data. In fact, without prior processing and simplification of the network, results differ significantly between case studies. This paper presents a method for OSM dataset simplification as well as the theoretical and analytical reasoning behind it. The simplification is done by a series of ArcGIS workflows and algorithms. The outcome of this process is compared to an angular segment analysis (ASA) of a segment model, an Integrated Transport Network (ITN) Ordnance Survey data model and an OSM street network data model. The results show that a simplified version of OSM data is highly comparable to a segmented axial line representation and that such data sets constitute an appropriate alternative for situations where segment maps are not available, such as complex, large-scale regional models and cross-country comparisons. The simplification workflow is transferable to other cases and data sets and helps to overcome common problems while significantly improving computational time needed in the process.
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Proceedings of the 11th Space Syntax Symposium
Space Syntax Laboratory, The Bartlett School of Architecture, UCL
The application of volunteered geographic information has rapidly increased over the past
years. OpenStreetMap (OSM) forms in this context one of the most ambitious and promising
projects, providing consistent global coverage of street network information. With a constantly
growing number of participants and the implementation of governmental and proprietary
based information is a complete coverage of global street networks within reach. The data
allows comparative cross-country analyses and any method developed within its framework
are transferable to other cases. This makes OSM a powerful and desirable data source for
applied network analyses, such as space syntax. However, OSM data does not come without
obstacles. Inconsistent representation of space, topological fragmentation and accuracy are
just some of the problems that one faces when employing OSM data. In fact, without prior
processing and simplication of the network, results dier signicantly between case studies.
This paper presents a method for OSM data set simplication as well as the theoretical and
analytical reasoning behind it. The simplication is done by a series of ArcGIS workows and
algorithms. The outcome of this process is compared to an angular segment analysis (ASA)
of a segment model, an Integrated Transport Network (ITN) Ordnance Survey data model
and an OSM street network data model. The results show that a simplied version of OSM
data is highly comparable to a segmented axial line representation and that such data sets
constitute an appropriate alternative for situations where segment maps are not available, such
as complex, large-scale regional models and cross-country comparisons. The simplication
workow is transferable to other cases and data sets and helps overcoming common problems
while signicantly improving computational time needed in the process.
Volunteered Geographic Information, Open Street Map, ArcGIS, Space Syntax, Street Network
The aim of this paper is to present a workow and methodology that allows the use of
OpenStreetMap (OSM) data in space syntax angular segment analysis (ASA). The reasoning
behind employing such data sets is the increasing scale of analytical investigations in the
context of space syntax. This augmentation of scale has become particularly necessary due
to the extensive global growth of cities and their urban hinterland into large complex urban
regions. These urban structures are simply too vast to be mapped manually or generated by
automated algorithms. This has created a situation in which the time and economic feasibility
of traditional as well as algorithmically derived axial line maps needs to be revisited. Previous
research proposed to make use of governmental so-called road-centre line data as an alternative
for a segmented axial line, more commonly referred to as segment maps (SM). However, very
little has been said about the disadvantages of such approaches particularly when global
Proceedings of the 11th Space Syntax Symposium
comparability is needed, something in which space syntax is believed to be particular strong.
OSM road-centre line data, on the other hand, I will argue, forms not only an appropriate
alternative basis for models in these situations, but it also allows global comparability as well
as being freely accessible on a large scale. Nevertheless, OSM data does not come without
disadvantages either. Particularly concerning excessive information in such data sets, which
makes a simplication prior to any ASA application necessary, caution needs to be exercised.
This paper consists of three parts; the rst revisits the foundation of space syntax axial line
models and the sequentially developed analytical method of ASA and its segment map (SM)
model. An emphasis is placed on the model underlying the analysis and the diculties arising
in the model generation generally and in large-scale applications particularly. In this light,
volunteered geographic information and governmental road-centre line data, such as the
British Integrated Transport Network (ITN) are reviewed as alternatives for SM models. Finally,
advantages as well as disadvantages of OSM data are discussed and the eect of these on ASA
The second part presents the structure and particularities of the previously introduced OSM
data, as well as the diculties researchers are facing when employing such data in ASA. I
discuss the three main diculties, which are topological inconsistency, trac management
components and excessive or redundant nodal information. I propose dierent GIS strategies
to simplify and remove such redundant information and explain the theoretical reasoning
behind them. The result is a newly derived simplied OSM network model, termed ‘SIMP’.
The third part evaluates the new SIMP model against OSM, ITN and SM models in ASA. I do this,
using descriptive statistics, visual comparisons, as well as a Pearson and Spearman correlation
analysis. The results show an overall high correlation between the four models, conrming
previous ndings. The new SIMP model exhibits higher correlations with the SAL model than
both OSM and ITN network models, indicating that a simplied OSM network does not only
form an appropriate alternative but one that presumably incorporates fundamental network
characteristics of SM models.
Axial analysis forms one of the fundamental techniques of space syntax. At the core of an axial
analysis methodology lies the axial line map, a representation of the continuous structure of
open spaces in urban settings. The rst axial line model was introduced by Hillier and Hanson
(1984, p. 17) during the early 1980’s and dened as a system of fewest and longest intersecting
lines covering all open spaces. These lines are the result of a two-step process where the spatial
system under investigation is rst represented through a two-dimensional organisation of
convex spaces. Convex spaces are polygonal representations of continuous open spaces, in
which each part of a space must be visible from every other part. The underlying rule for drawing
a convex space is that each polygon must feature the best ‘area-perimeter ratio’, starting with
the ‘fattest’. In a subsequent second step, this system of convex spaces is covered by a one-
dimensional set of axial lines. Axial lines are linear representations of longest lines of sight and/
or movement. Each convex space must be covered by at least one axial line, while each line
needs to be the ‘longest straight’ line possible (ibid., p.17).
Although Hiller and Hanson describe this process as reproducible and objective, there is
some discussion and ambiguity about the comparability and making of axial maps. Problems
arise for instance with dierences in the level of detail or resolution in which convex spaces
are produced, as this impacts the number and distribution of the resulting axial line map.
Problems also arise with the diculty to arrive at comparable reproducible solution for the
same given urban context. Peponis et al. acknowledge in this regard ‘SpaceBox’1, a software
that automated the generative process of convex spaces, but they criticise the mathematical
1 SpaceBox is a software developed by Sheep Dalton (1988) and includes several space syntax related functionalities
one of which being the generation of an all convex space map. The software’s partitioning algorithm extends a
walls surface area collinear until the produced line reaches another wall surface. See Carranza and Koch for more
recent work on convex spaces (2013).
Proceedings of the 11th Space Syntax Symposium
rigour of its computational algorithms to generate convex spaces (1997, 1998). According
to Peponis et al. neither the initial principle of generating convex spaces based only on an
economic partitioning, nor the extension of surfaces to the next opposite wall is a sucient
method. Both lead to multiple, conicting solutions, implying that a more sophisticated set of
rules is necessary. Interestingly, although the methodology of convex spaces is thought of in an
urban context most of the discussions are set in the context of buildings. This might be due to
the time-consuming process of producing convex spaces for entire cities, with the sole purpose
of deriving an axial line map. The scale of the area under investigation and respectively the
time necessary to produce such convex representations is certainly one of the most important
inuencing factors.
Moreover, Desyllas and Elspeth argue that not only the production of convex spaces, in general,
is dicult, but that it constitutes a ‘mathematically impossible problem’ to link all maximal
convex spaces with axial lines in an identically repeatable manner (2001, p. 27.6). The core
problem here is that there are several solutions to axial lines that full the criteria of being the
longest as well as covering all convex spaces (Batty and Rana, 2004; Ratti, 2004). As a solution
to this technical and theoretical problem Turner et al. (2005) – building on an initial but not
ideal solution from Peponis et al. (1998) – proposed an automated methodology that produces
a fewest line axial map. The starting point of their method is vector information of open space
boundary polygons. Based on this, a so-called ‘all-line map’ is generated (Penn et al., 1997).
The ‘all-line map’ is a map that features all lines that connect each vertex of boundaries and
buildings with all other visible vertices, i.e. all possible lines of movement. In a following step
Turner et al. employ an algorithm to reduce this ‘all-line map’ to a fewest line axial map. Their
results are reproducible and strikingly similar to the original Hillier and Hanson axial map (2005).
However, his method of the fewest line axial map generation, does not constitute an appropriate
way to produce models for cities and regions. There are two primary factors, which prevent the
application in a citywide and regional context. The rst starts with the source of data and its
denition of open space, a problem that the very initial convex space methodology already
inherited. What to include and what to leave out in a graphical representation of the real world
is left to the individual cartographer or researcher and forms core challenges in comparative
cartography and map-making in general. This challenge is of particular importance when
investigating suburban or rural areas. Suburban and rural areas often lack a continuous urban
form and hence a given limitation for movement and visibility. Consequently, the denition of
what can be considered an ‘accessible open space’ becomes vague. A problem that researchers
are also facing in the context of developing countries exists as roads are often not solidied
and boundaries between public and private spaces are less established. In these cases, an
alternative could be to rely on other sources of geographic data of open spaces that follow
precise denitions. Such sources are for example governmental agencies for cartography,
geodesy and planning or volunteered geographic information, both of which have precise
denitions of what and how open spaces are mapped.
Computational time constitutes the second diculty. With a rising number of mapped open
space polygons and their vertices, the necessary computational time to generate the fewest
line axial map increases as well. Turner et al. give an account of the computational time needed
for their algorithm to compute fewest line axial maps. A model of the small town of Gassin
took 119 seconds to compute and featured 5217 lines in its initially generated all-line map and
38 axial lines in the nal result (ibid.). Thus, the computational process for an entire city or
even a region, with far more than one million street segments will take signicantly longer2.
While theoretically the algorithm could run for any time needed, in praxis this is limited by the
software design dealing with large data sets. Currently the most commonly used software for
this is depthmapX. Initial tests using the software on large urban systems generating fewest line
axial maps have consistently produced application crashes. Varoudis et al. state the maximum
number of segments that can be computed by depthmapX as <1.500.000 (2013), resulting in an
axial line map of approximately 15000 lines. This makes an automated generation of axial lines
for a metropolitan or regional system at the time not possible.
2 The total number of axial lines in cities with a population of 300,000 can range between 10,000 and 15,000.
Proceedings of the 11th Space Syntax Symposium
Initially, the focus of axial line maps was to have a tool that allowed understanding complex urban
systems in a simplied comparable manner. Over time the primary use of this morphological
descriptive tool was to be found in investigations into the deep relation between human
behaviour and space. From the development of the methodology, throughout the last 30 years,
researcher have consistently found correspondence between the topological relationships of
spatial systems and pedestrian movement (Hillier et al., 1993; Penn et al., 1998; Desyllas and
Elspeth, 2001; Hillier and Iida, 2005) as well as vehicular movement activities (ibid.; Turner,
2005; Law and Versluis, 2015; Serra, Hillier and Karimi, 2015) and even global transportation
networks (Hanna, Serras and Varoudis, 2013). This is particularly the case since the introduction
of ASA in space syntax as an extension of axial analysis (Turner, 2001). The emphasis thus shifted
from a theory and tool to analyse spatial congurations to one of predicting the potential of
human behaviour in the form of movement and ows. Four studies focus on alternatives that
constitute possible models for an analysis of movement and ows in the build environment:
The pioneering work by Thomson (2003), Dalton et al. (2003), Turner (2005, 2007) and following
up on these studies most recently the work by Dhanani et al. (2012). All authors investigate the
possible application of dierent types of so called road-centre line data. The reasoning is that
their approach relies on replacing a segment map, which is used in angular segment analysis
rather than the in traditional axial line model the SM is based on. This study will follow the path
taken by the above named researchers and base the comparison on a segmented axial line
model, rather than emulating an axial line model, which inevitably will later be segmented in
order to perform ASA.
Road-centre lines ideally represent the geographic centre of the public rights of way network,
a transportation network of all paths on which the public have a legally protected right to pass
and re-pass. These transportation networks are based on vector line information and can be
generated through a variety of GIS methods such as automated processes of on ground collected
GPS data, generative processes based on cadaster boundary data or manual tracing of roads on
aerial photographs. In a subsequent step, additional information can then be attributed to this
line information such as road names, road type, travel direction, road geometry information as
well as a large variety of other possible attributes.
This makes road-centre line maps a powerful tool for a variety of GIS based applications. The
ones applied the most are transportation modelling and navigation routing. Road-centre line
data was rst provided by local governments, such as the TIGER3 data set by the United States
Census Bureau or the ITN4 by the British Ordnance Survey, as well as commercial companies,
such as the Dutch Company TeleAtlas5 or American-based Company Navteq.6 The latter provides
mainly line-based data for navigational systems. With the rise of the Internet and Web2.07,
publicly accessible road centre-line information became largely available through dierent
sources. The most predominant sources are Google maps and Bing maps, both available under
restricted license for non-commercial usage. In contrast to governmental and proprietary
based information with restricted license stands volunteered geographic information (VGI).
VGI describes all geographic data, which is created, assembled and disseminated voluntarily
by individuals (Goodchild, 2007). Open source VGI projects such as OpenStreetMap (OSM) and
MapQuest are available under a GUP license and hence freely accessible to anybody. Due to the
increasing number of online participants all over the world these projects are on the rise and
establish a commercially as well as academically meaningful alternative.
3 TIGER is an acronym for Topographically Integrated Geographic Encoding and Referencing and an American
based format used by the United States Census Bureau to describe land attributes such as roads, buildings,
rivers, and lakes, as well as areas such as census tracts. The TIGER format forms a base for the US part of the
OpenStreetMap project.
4 The Integrated Transport Network, is part of the OS MasterMap and a format provided by the United Kingdom
governmental Ordnance Survey.
5 TeleAtlas is since 2008 wholly owned by navigation system company TomTom.
6 Navteq is since 2011 fully merged into NOKIA.
7 Web 2.0, is a term describing the state of the Internet as a collaboration focused information platform, where
the user produces content. The term is set against Web 1.0, where content was provided as ‚ready-to-use’ and no
interaction with the user was aimed (O’Reilly, 2005).
Proceedings of the 11th Space Syntax Symposium
In the context of space syntax analysis 2003, Thomson (2003) pioneered when proposing to
make use of street networks. His study focuses on theoretical and technical problems based
on the model construction rather than an investigation on how dierent models eects the
analysis. In the study, he highlights possibilities of generalizing road networks. Simultaneously
Dalton et al. propose to make use of TIGER data and present initial results of their analytical
work (2003). TIGER is a data format only used in the United States providing road-centre line
information among other geo-referenced spatial data. Dalton conducts a fractal analysis and
compares a TIGER dataset with a traditional hand-drawn axial map of Downtown Atlanta, US.
He highlights dierences in the results of both models and concludes that the result is caused
by the very dierent representation of space. While a long linear avenue with adjacent side
streets is represented by one long axial line in a traditional axial line map in the TIGER dataset
road centre-lines are segmented by nature and have a node at each intersection (this is the
case for any road centre-line map). Any topological investigation would thus lead to a highly
skewed outcome. Moreover, Dalton raises the theoretical problem of radii, emphasising the
need for a ‘relativisation’ due to the dierences within each system (ibid., p.9). While Dalton
did not propose a solution to the problem his argumentation led to a series of investigations by
Alasdair Turner.
In his study from 2005 Turner presents a methodology that overcomes this problem of
segmentation and ‘relativisation’ by drawing on advantages of space syntax applying ASA to
road centre-line maps in combination with a segment length weighted algorithm. The results
of his 2005 and 2007 study indicate that metric radii in combination with weighted choice
measures present not only a suitable alternative to SM models but, in fact, generate better
correlations with ow data in the tested case studies. Turner emphasises that his measure holds
congurational information while incorporating plausible cognitive and physical constraints
(2007, p. 553). Turner’s ndings are reasonable since road centre-line maps are fundamental
representations of the accessible – rights of way – movement network and incorporate more
detailed angular information than axial line models.
Dhanani et al. (2012) follow Turner’s ndings and conduct a comparative study of an axial line
model and two dierent types of road centre-line based models. As mentioned previously,
there are dierent sources for road centre-line maps. Dhanani et al. studies’ focus on two
very particular networks: the governmental ITN data set and the OSM VGI data. Their studies
aim to understand whether a VGI-based data set constitutes a reliable alternative compared
to governmental data sets in the light of space syntax analysis. Beside of Dalton’s (2003) and
Turner’s (2005, 2007, 2009) work, there are no other comprehensive studies where space
syntax measures are applied to governmental road centre-line data sets correlating results
with empirical data. This is surprising as both of the studies rely either on the American TIGER
data or the British Ordnance Survey data sets. The diculty here is that governmental road
centre-line maps are presented as a reliable and coherent source of data, yet, this is only true
for information within one data set 8 and very little is being said about their comparability in an
international context.
Dierences occur between governmental data sets not only on an international level but also
within countries. The British Ordnance Survey for example provides three dierent road centre-
line data products: the OS MasterMap layer Integrated Transport Network (ITN) layer, the OS
Open Roads layer and the Merdian 2 layer. All these data sets provide comprehensive road
network information and are designed for routing and road network analysis, yet, their level
of precision and coverage diers.9 This means that the total amount of nodes and coverage of
real world details such as roundabouts are not the same throughout the three data sets. More
importantly such data sets are not available in every country. Germany, Italy and France–to name
only some–do not provide freely accessible data sets. This is why, the question of comparability
needs to be answered and investigated for each country individually and alternative sources
8 It shall be noted that errors do occur in governmental data sets as well, but they usually follow a random
9 See
local.htm for further information on the data sets and examples of their application.
Proceedings of the 11th Space Syntax Symposium
need to be found. The lack of comparable data makes it dicult for international comparative
approaches making use of such data sets, particularly in the context of space syntax.
In the light of this lack of comparable data, OSM data becomes more interesting as an
appropriate alternative to a segment map representation, which, in theory, provides a
comparable representation of space all over the world. OSM data is produced according to
a guideline indicating the level of precision and the handling of particular situations such as
divided highways, roundabouts, intersections or bridges (OpenStreetMap Wiki contributors,
2016). This makes the data, in theory, globally comparable. However, dierences in terms
of data quality arise due to the nature of its production and its contributors’ heterogeneous
understanding of street networks.
Understanding such dierences in quality is a non-trivial task in the realm of OSM data. There
is a set of ISO standardized quality measures to assess the quality of map-based VGI (OSM)
data. These measures are of particular interest for routing and navigation application, namely
positional accuracy and topological consistency (Senaratne et al., 2016, p. 6) and thus for a
space syntax application. Positional accuracy is a quantiable value reecting the dierence
between a mapped location and its real world location while topological consistency measures
how well topological relations (‘disjoin’, ‘meet’, ‘overlap’ or ‘equal’) are mapped. A simple
example for low positional accuracy would be a mapped intersection, of which the GIS location
is 20 meter further in the North than in reality. An example for bad topological consistency of
an intersection would be the case, in which two streets, which in reality are connected and
should share a common node, would not do so in GIS. To evaluate the two mentioned quality
measures it is necessary to compare the data set under investigation with the real world. This
is usually done by comparing the VGI data with ground-truth data. Ground-truth means data
that represents the respective exact location in reality. This is a theoretical value, rather than
an actually achievable goal for most GIS data sets. GPS systems feature on average a positional
accuracy of 6-10 metres to ground-truth. The ordnance survey MasterMap ITN data states its
positional accuracy with 1 metre in urban and 6 metres in rural areas against ground-truth.
Throughout the past decade, several authors have conducted comparisons of volunteered
geographic information with governmental as well as commercially produced geographic
information (Flanagin and Metzger, 2008; Neis et al., 2010; Zielstra and Zipf, 2010; Ludwig,
Voss and Krause-Traudes, 2011)10 to measure their quality. In the context of road centre-line
information the work by Mordechai Haklay was one of the rst to evaluate the quality of OSM
data (2010). Haklay used the British OS Merdian 2 road network as control measure to test
OSM data quality, his ndings indicated highest mapping qualities in urban and auent areas
and the lowest coverage in rural and poorer areas while positional accuracy ranges from over
70% to occasionally drop down to 20% (ibid., p.700). Overall OSM data covered 29% of England
based on a network from March 2008. In a subsequent study conducted in October 2009 this
percentage was already corrected to 65% of coverage (Haklay, 2009). This indicates a growth
of the network coverage by 36% within one year. Another study by Neis et al. (2011) dealing
with the case of Germany, compared the OSM network against the proprietary data set of
TomTom (formerly TeleAtlas) and estimated a complete coverage of the German OSM data by
the year of 2012. Moreover, already in 2011 the OSM data exceeded the topological consistency
and completeness of the TomTom network by 27% including pedestrian path ways (ibid.).
The continuous growth and its pace of the OSM data set, does not only make a coverage and
quality assessment dicult, but indicates that it is only a matter of time that full topological
consistency will be reached. The number of total users in the OSM community as well as their
nodal contribution to the network shows a growth of the total user number to 2,9 million since
the start of the project 2004 and gives insights in the pace of this process.
10 See Sehra et al. (2013) and Senaratne et al. (2016) for a comprehensive review of studies dealing with quality
assessment of VGI data.
Proceedings of the 11th Space Syntax Symposium
Figure 1 - Visualizing road updates. All roads shaded by how recently they have been updated by users.
Older imports are in green and blue, while cities with strong and active communities and the eect of
recent automated editing makes areas glow red. (2013) Source:
(retrieved on 1 August 2016)
Hakley et al. (2010, p. 11) investigate how many volunteers are needed to map an area thoroughly
concluding that areas mapped by more than 15 contributors per square kilometre feature a very
good positional accuracy of below 6 metres for resulting VGI data. In regard of the growing
numbers of contributors this leaves us to expect an equal rise in topographic consistency and
positional accuracy. An additional positive eect to the coverage of areas, beside the growing
number of contributors, is the fact that governmental agencies increasingly provide their
data for public usage. Likewise, are the American TIGER network as well as the AND Dutch
road network fully implemented in the OSM network, aiding not only to the coverage but
positional accuracy of the OSM data set. A visualised snapshot of the data and its topicality
reveals updating intervals, as well as showing that Great Britain and Germany are part of the
best-mapped countries of the OSM project (Figure 1). All of the above studies use ground-
truth data for the evaluation of VGI quality. Still, such data is not available in every country and
more diculties for the assessment of VGI data arise due to the lack of ground-truth data for
comparison (Senaratne et al., 2016, p. 6). To overcome this lack of ground-truth data, Keßler
and de Groot (2013) propose a method to indicate quality of VGI via trust assessment models.
Their approach is based on a trust assessment model of the independent contributions in an
OSM data set. Albeit presenting promising results, the methodology is at an early stage of
development and does not propose an applicable method for the eld. At the present stage,
this leaves the research with as-good-as complete network for some countries with reasonably
accurate precision, but a manual control of the entire data set by the researcher stays a
necessity. With regard to future research the OSM will very likely constitute the most coherent
freely available data set.
Dhanani et al. (2012, p. 30), assess the usage of OSM in space syntax to be problematic and
describe the data as lacking ‘of consistency [,…] accuracy and coverage’. Their study calls on
researcher to rely on governmental data such as the British OS MasterMap ITN, yet, as mentioned
earlier, as data is not accessible in every country and level of detail diers throughout dierent
data sets, this approach remains unsatisfactory: The OS MasterMap ITN network covers only
the vehicular network disregarding any path or street that is only accessible to pedestrians.
The resulting vehicular centred spatial representation can therefore only be used to evaluate
vehicular structures. Space syntax segment map representation on the other hand sees space
through the eye of an individual moving in space and constitutes a sharp contrast to a vehicular
Proceedings of the 11th Space Syntax Symposium
only street network. There are also other diculties within the ITN data set that render an ad
hoc use impossible. Dhanani et al. note that the ITN network comprises all trac management
features including trac islands, articial cul-de-sacs or roundabouts (ibid., p.6). According
to the authors, using such data creates a ‘disjoint and fragmented network’ particularly if a
researcher is interested in other modes than a purely vehicular estimation. The usage of such
data is not recommendable without any prior processing. Prior processing is also necessary for
OSM data making it indispensable to develop a strategy to overcome said inconsistency and
arrive at a comparable network for any given case.
The following section gives an overview of the necessary components to create a road network
based on OpenStreetMap data and the necessary steps of post processes to allow an application
in space syntax ASA.
At present, OSM data sets are divided into four dierent elements: nodes, lines, surfaces
and relations. For an ASA only line information is necessary, but not all of the available line
information and categories are useful. The OSM wiki provides extensive accounts on all dierent
key categories and their morphology (OpenStreetMap Wiki contributors, 2017), it is important
for each researcher working with OSM data to make him/herself familiar with all categories and
morphologies. Decisions about which category to exclude might dier for example in cities in
developing countries. The following steps should to be considered as a general guidance: For
the purpose of network analysis only components with the key highway=* shall be used. This
key denes any kind of road, street or path and their respective importance in the network
hierarchy (from the most important ‘motorway’ to the least ‘service’) and, thus, gives a good
account of the rights of way network. The following list assess which are recommendable to be
included in a network for an application in ASA: highway=motorway; trunk; primary; secondary;
tertiary; unclassied; residential; motorway_link; trunk_link; primary_link; secondary_link;
tertiary_link; living_street; pedestrian (ibid.). Particular care needs to be taken with the key
pedestrian as it includes pseudo polyline information of squares and these need to be cleaned
and subsequently broken into individual segments. Other sub keys such as highway=service;
path or bridleways can be included but are not recommended, as they are of very small scale
and might otherwise be eradicated in a subsequent simplication process.
With a view to this selected data there are three main diculties that occur when applied in a
space syntax context.
1. Topological inconsistency occurs if street segments are supposed to share a connecting
node but due to positional inaccuracy fail to do so. This is often the case at intersections
of dierent contributors. Even a small gap between two nodal ends of 1 cm can create
a network fragmentation. It is, therefore, necessary to process and clean the data from
these inconsistencies.
2. Trac management components are network details that are necessary for vehicular
trac management but have no immediate impact on cognitive route decision-making.
Such details are for example roundabouts, small trac islands or motorway trunks.
Ideally roundabouts are simplied into simple intersections whereas meandering trunk
links are represented by single links. Moreover, this is also the case with regard to dual
line representations. Space syntax analysis is a non-directional approach in the sense that
the possible travel directions are not taken into consideration and each space is treated
as equally accessible. A dual line representation constitutes only a reasonable option if
directions are taken into consideration. Hence, the model needs to be cleaned from said
dual line representations.
3. Redundant or excessive nodal information are often problematic when using OSM data.
Although the OSM guide notes that nodes should be used in an economic manner,
contributors often have dierent interpretations of what ‘economic’ means. This is
particularly the case for curved roads, but also occurs on straight lines. Ideally each street
is simplied to its fundamental segment.
Proceedings of the 11th Space Syntax Symposium
In order to overcome these diculties a series of GIS algorithms have been developed. The
following proposed solutions are employing the GIS software ArcGIS Desktop 10.2 from
Esri. I employ ArcGIS because it is the only software that provides solutions for all three said
diculties. At present, only a few of the solutions presented here can be achieved with open
source GIS software packages. Due to the scope of this paper only a brief description of the
applied core functionalities will be given. Figure 2 shows a workow diagram for the proposed
solutions, while Figure 3 gives an illustration of each obstacle and its favoured solution after the
application of the simplication method presented here.
Figure 2 - Workow of ArcGIS tools and algorithms to solve: 1. topological inconsistency; 2a. dual line re-
moval; 2b. road detail removal and 3. line simplication.
Proceedings of the 11th Space Syntax Symposium
Figure 3 - Illustration of each diculty found in OSM data: 1. topological inconsistency; 2a.
dual line removal; 2b. road detail removal and 3. line simplication as well as the condition
after application of the simplication method.
1. Starting with the approach of solving topological inconsistency (Figure 2:1), it should
be mentioned that a lack of network information, such as entire missing streets cannot
be solved through automated processing and that the OSM data needs to be carefully
checked by the research prior to any post-production. More so, this is a strategy to
overcome small inconsistencies that are dicult to identify manually. The proposed
process reconnects topological inconsistencies by a given tolerance distance and in
a subsequent step merge segments that can be considered as independent streets
(from intersection to intersection) together. This will leave the researcher with a street
network of real segments and consistent topological information. The two core ArcGIS
functionalities the workow is based on are ‘integrate’ and ‘unsplit’.
Proceedings of the 11th Space Syntax Symposium
The integrate tool is applied to extracted nodal information, rather than the actual
line information, to overcome misalignment at intersections. Integrate maintains the
integrity of shared nodal feature information by making features coincident if they fall
within the specied x, y tolerance. Features that are considered identical or coincident
are merged. In a subsequent step the newly generated nodal point information is used as
a basis for a snap command of the initial street network. This will consequently connect
lines, which feature topological inconsistencies, at a new point based on the location of
their nodal line ends.
The unsplit tool is then applied to the now topological consistent line network. The aim
is here to aggregate single part line features into multipart features in order to arrive
with continuous street segments. Unsplit merges lines that have coincident endpoints.
This can be done by relying on any given attribute information or, as in this case, solely
by geometric relationships. Merged lines are of particular importance with regards to
further simplication processes.
2. The next diculty is the existence of trac management details and dual line
representations in the data sets (Figure 2:2a & 2b). Not only do such details (roundabouts,
trac islands, etc.) create dierences in angular movement, while the general journey
direction stays the same, but more importantly they increase the total number of journeys
(dual line highways) and skew analytical results towards an emphasis of such details.
Especially in the light of none directed centrality analysis dual lines make little sense. This
could be negligible if trac management details were normally distributed throughout
the street network. However, this is, not the case with most examples and particularly
not with inter-city and regional scales. There are four main ArcGIS components, ‘merge
divided roads’, ‘collapse dual lines’, ‘collapse road details’ and ‘integrate’ that help to
remove such dual lines and reduce low-level street network complexity.
The merge divided roads is an algorithm that merges road segments, which are parallel
along a signicant distance into a single centre line. The merging process is based on
common attributes that can be computed on the basis of the initial highway keys. It is
fundamental that the merge eld parameters are established properly to avoid conicts
during the process. The divided roads algorithm can be applied to entire data sets and
maintains topological relations with adjacent streets.
The collapse dual lines to road centerline is an algorithm designed to derive with centre
lines from a base of street perimeters. It is, therefore, a less sophisticated form of
simplication and it is not recommended to perform the algorithm on large datasets
including multiple-lane highways with interchanges, ramps, overpasses and underpasses.
In individual cases where the merge divided roads tool does not arrive with satisfactory
results, the collapse dual line to road centerline tool can form a useful alternative.
The collapse road detail, on the other hand is an algorithm that depicts small road segment
details and open congurations that interrupt the general trend of a road network and
collapses or replaces them with a simplied feature. The collapse distance on which the
tool performs is dened by the maximum size of the largest road detail and can dier for
each model. If the collapse road detail tool does not solve or remove some of the details
the integrate tool explained earlier constitutes an appropriate alternative. Particular care
needs to be taken when using integrate on road details as it can impact the topological
consistency of the data and should hence not be performed on entire data sets but single
3. Line simplication is usually applied when segment records feature far more data than
necessary for computer analysis or visual representations (Figure 2:3). In the case of
space syntax and the use of VGI street networks this poses a conceptual question aside
of excessive data. While road-centre lines depict the centre of the road an axial line
(as base for a segment map line) is based on the longest line of sight. A generic street
usually features a much larger eld of vision than that of a single line. While axial lines
fundamentally connect convex spaces these lines naturally pervade more than one space
Proceedings of the 11th Space Syntax Symposium
at once. Road-centre lines on the other hand simply represent the centre of the road
and, therefore, feature excessive angular information that does not impact the eld of
vision or accessibility and, thus, has no eect on the actual movement in space. This is
why, a removal of such road details should be based on the eld of vision of each street,
i.e. the street width. Since road-centre lines give a precise account of the centre of each
street segment a simplication process should allow the newly generated feature to
deviate to at least the extent of the eld of vision. Such processes can be performed by
the Douglas-Peuker Algorithm (DPA) (1973). The DPA is broadly considered to deliver the
best perceptual representations of the original segment and generates new segments
based on a deviation tolerance. In ArcGIS this can be done by applying the simplify line
The simplify line tool reduces and removes redundant nodes of line features. Among
others, when applied with the POINT_REMOVAL functionality it employs the DPA. The
aim of the algorithm is to extract the essential segment form based on a previously
selected o set tolerance. The strength of the algorithm is its reproducibility and process
speed, and that it arrives at the same solution to the same given problem.
If the above steps of the methodology are followed the simplied version of a road-
centre line map (SIMP) looks visually as well as topologically much closer to an axial line
In order to test if the theoretically laid out version of a simplied OSM network (SIMP) constitutes
a comparable alternative to a segmented axial line map and is, thus, suitable for the purpose
of analysis of dierent scales and very large ones in particular the model will be analysed and
correlated with results from an ASA of a segment map, ITN and OSM model. The comparison
extends and the builds on methodologies by Eisenberg (2007), Turner (2007) and Dhanani et al.
Eisenberg (2007, p. 5) focused on comparison of dierent axial line models for the same cities.
The dierent models that Eisenberg compares are developed as a by-product of variations
in analytical scales (pedestrian, bicycle and vehicular) and variations in the detail of the base
information used for the production of the axial line maps. Eisenberg highlights that three
indicators are of interest for a comparison. First, the impact of base map scales; Second,
dierent levels of detail; And, third, dierent city morphologies (ibid., p.5). All aspects are
directly transferable to the dierent network models previously introduced. Eisenberg’s
ndings suggest that the analysis should focus on ‘rank correlation measures’ in order to have
a meaningful comparison (ibid., p.8). Eisenberg’s ‘rank correlation measures’, are applicable
to every kind of network representation. This measure simply compares values and their
respective rank within the data set. With Eisenberg’s measure an appropriate method for the
aimed analysis is established where numbers of lines dier signicantly and the resulting values
do not form a comparable unit.
In addition to ‘rank correlation’ this comparison will draw on the methodology of Turner (2007).
Turner proposed an angular based analysis in combination with segment length-weighting and
the introduction of a metric length based radius. While an angular based analysis incorporates
the cognitive dimension of route choices, the reasoning behind a segment length-weighting is
to overcome the large dierences in segment numbers between the dierent representations
(ibid., p.541). Turner shows how his propositions are an advancement for space syntax analysis
in general and in the context of road-centre line networks in particular.
Finally the above proposed methods will be merged with a methodology by Dhanani et al.
(2012). Dhanani et al. conducted a comparison of road-centre line networks against axial line
models using a general description of the network characteristics followed by a topological
and metric step depth analysis from the most central segment. Although the outcome of the
topological step depth showed interesting results the application of topology on a road-centre
Proceedings of the 11th Space Syntax Symposium
line network remains inappropriate as road-centre lines topological information is highly
skewed by its nodal information. The measure of topology in space syntax analysis is based
on the cognitive and visual space in the sense that what is considered as one space in space
syntax would result in several spaces in a road-centre line network. The analysis will only draw
on the measure of metric step depth (MSD) for comparisons as MSD is not aected by nodal
In summary, the following comparison is based on four dierent road network models of the
centre of the city of Leeds. The city of Leeds was selected because it features a variety of dierent
network details such as motorways, trac management details as well as local paths. The road
network models are: the Ordnance Survey ITN network, the OSM network, a simplied version
of the OSM (SIMP) and a segmented axial line model (SM). The ITN network and the OSM
data are not simplied but instead used as they are provided by the organisations. Moreover,
the ITN and OSM networks where controlled on topological consistence, yet, no irregularities
were found. Some network categories, as those mentioned in the OSM data sections, have
been removed from the OSM data set while trac management details remained unchanged.
The four models are compared in regard to their network characteristics and analysed on
14 dierent radii from 100 up to the entire system n11 using angular segment analysis with
segment length weighting. The models are analysed on closeness and betweenness centrality.
The resulting structures of three exemplary scales are visually compared. Then, subsequent
correlations are conducted using ‘rank correlation measures’. To facilitate comparisons mean
values of coincident segments of the ITN, OSM and SIMP with the SM model are plotted on
each respective SM segment.
11 The applied scales are: 100, 150, 200, 300, 500, 800, 1300, 1800, 2500, 3200, 4100, 5000, 6100 and n.
Figure 4 - Detailed section of dierent network models of ITN, OSM, SIMP and SM.
Proceedings of the 11th Space Syntax Symposium
Figure 4 shows a small section of each of the modelled areas. The section of the ITN network
shows trac islands as well as road interruptions. Some roads have signicant angular turns
just before their connection with the adjacent road. This is because for trac management
purposes rectangularity is preferred. In the light of angular segment analysis, Dhanani et al.
(ibid., p.10) consider this preference an important aspect and the most detailed and ‘optimal’
account of the street network. The aerial photo of the area (Figure 4) shows that at this point a
straight connection is a more reasonable account of the real world situation. Additionally, at the
lower right there is a road divergence into two separate lanes. A noteworthy detail is also that
roads, which could be considered as intersecting in reality do not share a common node in the
road network, due to a 5-10 metre distance of their road-centre.
Statistics ITN OSM Axial SIMP
Segments 15049 9308 5072 3908
Total length (m) 283410 276388 240534 238848
Computation time (min) 14.31 4.49 1.21 0.44
Table 1 - Network characteristics for each model.
Table 1 highlights the network characteristics for the four models and how they dier
numerically. The ITN network features the longest total network length with 283410 metres. This
is particularly due to the several roundabouts and trac management details within the model.
The comparison of trac management details with the length of the ITN and OSM networks
enables a rough account of the eect on the length of the network. This account does not come
to its fullest as the OSM network features streets and connections that are not represented in
the ITN. The several multi line motorway roads, which are represented by a single segment in a
segmented axial line and SIMP model cause a large dierence of 40km of the ITN and OSM data
in comparison to the segmented axial model. Comparing all networks, the dierence in number
of segments is striking. The ITN model has three times more segments than the segment map
representation. This dierence is due to the curved roads and roundabouts, which feature large
numbers of segments in order to give precise accounts on the length of the lines. While this
exemplies the detailed account on angular changes in road centre-line networks, it also shows
the inherent problem this data has when it comes to space syntax analysis. The computational
time is O(n2) to the number of segments. Generally speaking, the ITN and OSM are similar
in their measures and the dierence in number of segments is as expected. With regard to
the segmented axial line and SIMP model the question is whether the SIMP model, with 33%
less segments, does also stores less information. The number dierences can be explained
by the ‘cleaning’ of intersecting spaces: Whenever three segments intersect with each other
segmented axial line models tend to create clusters of very short segments. Additionally, when
the axial line model is converted to a segmented axial line, stubs that fall over 40% of the line
length are not removed and might also contribute to this dierence. The SIMP model features
almost the same length as the segmented axial line model pointing towards a similar degree of
spatial representation.
Proceedings of the 11th Space Syntax Symposium
These observations become more apparent with a look at the histograms for segment length
distribution for each m odel type. While the ITN network exhibits an even increase of segment
length with declining frequency, the OSM shows an initial increase indicating fewer stubs and
curve segmentation than the ITN. Moreover, the short line cluster eect of the SM model
becomes visible with almost thousand segments in the range of approximately 1-10 metres.
Contrarily, the SIMP model has a steep increase of frequency with a peak at a mid range of
approximately 30 metres indicating less of a short line information. The simplication range
used during the simplication process has an inuence on this peak.
Dhanani et al.’s (ibid., p.25) study shows that dierences between road centre-line network and
axial line models are consistent in their appearance and concludes that the dierent models do
not form a fundamentally dierent structure of the spatial conguration. In the next step I will
compare the new SIMP model with this assumption. Figure 6 shows the number of segments
for nine dierent radii where the maximum is 2,5km as this is the distance at which the entire
system was captured (in other words n). For the four models, the total number of segments
reached per metric distance increases in relation to the total number of segments. The semi-log
plot highlights these similarities and dierences, especially at lower scales. The SM and SIMP
model, exhibit a similar development, while the OSM and ITN, which were initially similar,
disperse towards growing metric distances and due to the increase of network details. Unlike
the values for the central segment the curve for the edge segment shows a slightly uneven
development. This becomes clearer in the semi-log plot of the data. Here, particularly the
development around the scale of 500 metres unveils that there are underlying dierences in
the complexity of the models that might have an eect on the analysis.
Figure 5 - Histogram of segment length distribution of each of the four models.
Proceedings of the 11th Space Syntax Symposium
In order to arrive at a better and more detailed account of the impact of dierences in the
network morphologies, I conduct a comparison of betweenness and closeness centralities
using a segment angular analysis with segment length weighting. The models are analysed on
14 dierent radii. The applied scales are; 100, 150, 200, 300, 500, 800, 1300, 1800, 2500, 3200,
4100, 5000, 6100 and n. Two of these scales, 800 and n, are visualised in order to understand the
geographic distribution of dierences. Figure 7 shows the results for betweenness centrality.
Figure 8 shows the results for closeness centrality. The values of each gure are broken down
using a quantile division. This is done to overcome signicant outliers in the data sets that make
a natural break highly skewed and the resulting maps illegible. These circumstances make
it necessary to process the data in a GIS programme rather than applying the implemented
symbologies of depthmapX.
Figure 6 - 1a: Number of segments for dierent metric step depth from the most central segment for ITN,
OSM, SM and SIMP models. 1b: Semi-log plot of the same data set. 1c: Number of segments for dierent
metric step depth from an edge segment for ITN, OSM, SM and SIMP models. 1d: Semi-log plot of the same
data set.
Proceedings of the 11th Space Syntax Symposium
Figure 7 - ITN, OSM, SIMP, and SM models analysed on ASA SLW betweenness centrality on radius metric 800
(1) and radius n (2).
Proceedings of the 11th Space Syntax Symposium
Figure 8 - ITN, OSM, SIMP, and SM models analysed on ASA closeness centrality on radius metric 800 (1) and
radius n (2).
Proceedings of the 11th Space Syntax Symposium
The results show that all models exhibit comparable patterns on all of the two visualised scales
and both measures of betweenness and closeness centrality. This conrms the initial ndings of
Dhanani et al. (2012). However, similarities in the results were much stronger between the OSM
network and the SM than they were between ITN and SM. Nominal segment dierences appear
to have a higher impact on betweenness centrality than on closeness centrality. Models with
large numbers of short segments and high degree of precision, such as the ITN network, are,
thus, more likely to be aected by outliers and unexpected clusters, than models with fewer
short segments. Moreover, the ITN network shows high values on all scales in the motorway
network. The SIMP model showed patterns that were visually stronger related to the SM model
than to the ITN or OSM and more similar to the OSM compared with the ITN. This is rather
unexpected as SM models are thought to be intrinsically dierent.
After getting an understanding of dierences and similarities in the geographical distribution
of the data between the dierent models and the SIMP model in particular a nal analysis of the
statistical extent of these observations is conducted. This will give an account of how models
behave in comparison to each other across all scales. As elaborated before, the analysis draws
on Eisenberg’s proposed ‘rank correlation measure’. To give a more detailed account, dierently
to Eisenberg, this analysis will compare all segments that are intersecting rather than only 10%
of highest values proposed by Eisenberg (2007). This is done by plotting mean values of the ITN,
OSM and SIMP on the SM model. The SM model is used as a base and comparisons are only
conducted with streets whose middle point falls into a 10-metre distance of a SM segment.
These middle points are then snapped to the closest segment and plotted on the SM model.
If more than one street segment of an ITN, OSM or SIMP model falls into this category, their
mean is calculated and plotted on the SM model instead.
Eisenberg’s rank correlation is based on Spearman’s Rank correlation (ibid.). Spearman’s Rank
correlation coecient is generally used to identify and test the strength of a relationship
between two sets of data. It tests if the relationship of both variables can be described by a
monotonic function. Ideally, the SIMP model could predict the segmented axial line model by
such monotonic function. In addition to this, a Pearson correlation will be conducted. Rather
than correlating the dierent ranks of each variable, a Pearson correlation works with the
actual values of the variables and measures their linear correlation. Both correlations provide a
coecient R2 indicating how related the variables are with each other. A coecient of 1 indicates
that the two models are identical. Any value below 1 describes the degree of dierence. One
can hence compare the dierences between all models statistically and provide a correlation
coecient to describe the tness of the SIMP model for the purpose of space syntax ASA. The
analysis is based on 14 dierent scales for both space syntax measures of betweenness and
closeness centrality. Figure 9 and Figure 10, show Pearson and Spearman correlations of ITN,
OSM and SIMP compared with the segmented axial model and, subsequently, the same for all
models correlated against the SIMP model.
Starting with Figure 9 the ndings from the initial visual description becomes also statistically
apparent. A rst observation is that the Spearman rank correlation provides more consistent
results across scales and measures with weaker dierences and higher scores. The Pearson
correlation on the other hand shows much stronger dierences in the four data sets but features
a signicant outlier on the scale of 100 metres for closeness centrality. In regard to the single
models the ITN model shows lower correlations across both Pearson and Spearman measures
and on both betweenness and closeness centrality. Particularly interesting is the signicant
drop towards higher radii, with a lowest correlation of 0,56 on Pearson for betweenness and
closeness. This increases at the Spearman’s rank, however, the general tendency towards
lower correlation at higher radii persist. In terms of the visual observations made earlier this
is caused by trac details and the strong representation of motorway features. The OSM and
SIMP model on the other hand show very comparable correlation developments. An exception
of this is the Persons correlation for betweenness centrality of the OSM model where similar
to the ITN a sudden drop at higher radii is visible. The SIMP model correlates stronger across
all measures with the highest scores of 0,983 for Spearman correlations of closeness centrality
metric 1300 and 0,919 for betweenness centrality. Contrary to OSM and ITN the correlations for
SIMP are very consistent.
Proceedings of the 11th Space Syntax Symposium
Figure 10 shows the Pearson and Spearman correlations for 14 dierent scales and closeness
and betweenness centralities. However, this time ITN, OSM and SM models are compared with
SIMP. The general correlation developments are very similar to the ones we have observed
previously, with a progressive drop of values towards higher radii. Interesting is at this point how
ITN and OSM behave compared to the SIMP model. While the ITN networks shows a slightly
weaker correlation, the OSM correlates much stronger. This was on one hand an expected
result, as the SIMP model is entirely based on the OSM. On the other hand in the light of the
overall comparison it seems as if the simplication process brought the simplied OSM model
much closer to the segmented axial line representation than expected.
Figure 9 - 1: R2 of a Pearson correlation for ASA segment length weighted betweenness centralities
(1a) and closeness centrality (1b) for 14 dierent metric radii (from 100 metres to n) for the three
dierent network models SIMP, OSM and ITN against the SM model. 2: R2 of a Spearman correlation
for ASA segment length weighted betweenness centralities (2a) and closeness centrality (2b) for 14
dierent metric radii (from 100 metres to n) for the three dierent network models SIMP, OSM and
ITN against the SM model (left). Correlation is signicant at the 0.01 level (2-tailed), N=3172.
Proceedings of the 11th Space Syntax Symposium
Figure 10 - 3: R2 of a Pearson correlation for segment length weighted betweenness centralities (3a)
and closeness centrality (3b) for 14 dierent metric radii (from 100 metres to n) for the three dierent
network models SM, OSM and ITN against the SIMP model (left). 4: R2 of a Spearman correlation for
segment length weighted betweenness centralities (4a) and closeness centrality (4b) for 14 dierent
metric radii (from 100 metres to n) for the three dierent network models SM, OSM and ITN against the
SIMP model (left). Correlation is signicant at the 0.01 level (2-tailed), N=3172.
These dierences become more apparent with regard to a log-log scatterplot of betweenness
and closeness centrality of the global scale n (Figure 11). The diagram shows a log-log scatterplot
of each of the measures allowing a visual comparison of outlier distribution within each data
set. The more dispersed the values are the less they correlate while linear consolidation implies
stronger correlations. This is clearly visible for the log-log plot of axial and SIMP while both
other models show stronger dispersion. The ITN model shows outliers across the values from
low to high, which is particularly the case for closeness centrality. To summarize, the results
Proceedings of the 11th Space Syntax Symposium
Figure 11 - Log-Log plots for the SM model compared to ITN, OSM and SIMP respectively for ASA SLW
betweenness and ASA closeness centralities on radius n.
Proceedings of the 11th Space Syntax Symposium
show that the four models dier especially in terms of the number of short length segments.
This dierence can be described by an exponential relation and has a signicant impact on
the computational time needed for the analysis. The results of the metric step depth analysis
conrm the ndings of Dhanani et al. (2012) and show that all models share a similar complexity
in terms of their nodal distribution. However, the analytical space syntax analysis showed that,
albeit, there is a similar distribution in the data in general the geographic location of these
dierences has an impact on the results. The ITN network is strongly inuenced by its emphasis
on vehicular movement and trac management details. This makes it less comparable to the
segmented axial line model than the OSM model or the SIMP.
Concluding, this paper elaborated the tness of OSM data in space syntax analysis, it proposed
an ArcGIS simplication workow and presented the theoretical reasoning behind the method.
The nal tness tests showed that the simplied OSM network (SIMP) exhibits very strong
similarities with the traditional segmented axial line model across all investigated cases. It
features the topological and angular information of the OSM network with the simplistic
representation of a segmented axial line model. This is rather surprising, because the alterations
in the model are mainly based on segment nodal reduction and minor topological alteration.
The Pearson and Spearman correlation analysis showed that the SIMP model is in fact stronger
related to the segmented axial model than to the OSM model. The strong similarity between
SIMP and segmented axial also poses question to weather axial line models are such intrinsically
dierent representations.
Overall the ndings suggest that a simplied OSM network forms an appropriate model for
space syntax analysis, particularly in the light of regional investigations where the production
of an axial line model is not a feasible option.
Proceedings of the 11th Space Syntax Symposium
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... These authors stressed that centrality measures correspondences change with different metric radii, and RCL could be adjusted according to analysis scale. Krenz (2017Krenz ( , 2018 addressed OSM RCL for ASA presenting methods of checking and adjusting topological inconsistencies, dual line and road detail removal, and simplification using georeferenced proprietary platform ArcGIS. For simplification Krenz mentioned threshold distances between 2 and 20 meters depending on hierarchy street width. ...
... In parallel with space syntax developments, the volunteered geographic information OSM project increased range providing open RCL models worldwide, becoming more accurate in depicting location and road connections (Dhanani et al., 2012;Krenz, 2017Krenz, , 2018Minaei, 2020), facilitated by volunteer work, collaboration and developing techniques based on aerial and satellite images and Global Positioning Systems (GPS) speeding updates (Husain & Vaishya, 2018;Liu et al., 2019). OSM RCL provides road labelling easing inter-modal analysis; OSM database also locates built facilities, facilitating studying city complex dynamics (Eldijk et al., 2020). ...
... Differently from some other space syntax RCL studies (Kolovou et al., 2017, Krenz, 2017, 2018, roundabouts were not removed and parallel lines were not joined. This decision followed a twofold reasoning: (i) to maintain automated RCL preparation steps and; (ii) existing permeabilities and barriers to movement. ...
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Axial and RCL (Road Centre Lines) models are street network representations used as base for space syntax Angular Segment Analysis (ASA) studies of centralities in cities. Although RCL is widely adopted for ASA, steps to treat RCL seem uncompleted. On the other hand, in Brazil many studies maintain axial models for ASA, despite standardization and updating limitations. Understanding models’ importance as it affects results, and in order to advance more confidently in studies, this paper compares axial and different modes of Open Street Map RCL ASA for two medium-to-large sized north-eastern Brazilian cities (Joao Pessoa and Natal). Closeness and betweenness centralities (i.e., measures of integration and choice) are studied for the city and their old city centres, as well as interrelations between centralities for different scales. Although all models work differently, they capture city centralities and scale dynamics, pointing towards natural urban voids in both cities accentuating routes movement hierarchies. Understanding some limitations in RCL simplifications proposed so far, a further procedure is presented that yielded good results in terms of route depiction and highlighting city centralities reading, while allowing more standardized and easily updated city models, especially relevant for rapidly changing Brazilian cities.
... While empirical studies based on the different syntactic measures abound, research on the effect of street modeling conventions on the different measures of directional distance is surprisingly lacking (Koch, 2019). The few studies that explicitly discuss the effect of modeling conventions on directional distance focus only on a single representation of street networks or a single definition of directional distance (Ratti, 2004;Feng and Zhang, 2019;Krenz, 2017;Dhanani et al., 2012;Eisenberg, 2007). To fill this gap, we compare three different ways of analyzing directional distance-namely, by number of steps, by number of direction changes, and by angular deviation-in the context of accessibility and reach analysis (Peponis, Bafna and Zhang, 2008;Feng and Zhang, 2019;Ståhle, Marcus and Karlström, 2005). ...
... As a form of crowdsourced, volunteered geographic information (VGI), it is freely available and has gained recognition within the space syntax community (Dhanani et al., 2012;Krenz, 2017;Gil, 2015). Swedish municipalities and governmental organizations regularly add data to OSM. ...
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Modeling spaces and their relationships is at the core of syntactic analysis, including reach analysis. In a syntactic model, two spaces can be described as close together or far apart based on the directional distance between them. In this study, we compare three different ways of measuring directional distance—namely, by number of steps, by number of direction changes, and by angular deviation—in the context of accessibility and reach analysis. By graphically showing how choosing a different way of measuring directional distance can result in a different reach or accessibility pattern, we provide an intuitive understanding of the different natures of the syntactic measures. By demonstrating how the modeling conventions and the geometric composition of lines at a local scale can have a huge impact on the results of syntactic analysis at a larger scale, we call for more attention to the conventions and principles used for modeling street networks.
... We first collect the basic geometrical data about the city using two sources of open Geodata, the street network from the crowdfunded OpenStreetMap and the buildings and plots from the Open Official Geodata of the state Thuringia, Germany. Then we merge both data as the difference in quality does not affect any later analysis of them (Krenz 2017). ...
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In this paper, we discuss a few methods for mapping the location of urban activities. We focus on establishing the geometrical relationship between the frontages of urban activities and their morphological context. Frontages act as the interface between the users of the public space and the activities within its surrounding buildings. Studying how this interface connects to the surrounding is not a trivial task. It could give different results if we use different units or other mapping methods due to a known error caused by two main effects of the Modifiable Areal Unit Problem; the zoning effect and the scale effect. The first is concerned with the choice of sizes for the analysis area, and the second is the mapping method applied to the analysis unit. We introduce in this paper a few methods to facilitate the ease of establishing relations between frontages data for spatial queries and statistical inferential. It provides a platform to connect between different urban activities and urban forms like retail activity to a street or accessibility value to a specific type of service. Using this method on the street-level activities data collected in Weimar, Germany, we demonstrate how to bring together more detailed information on the urban retail centres and their connection to the local built environment.
... weights. Re-modelling generally aims to remove elements which cause unwanted noise in the ASA (Krenz, 2017b), but its desirability ought to be evaluated case by case. In this study we assessed the appropriateness of simplifying the winding highway entrance and exit routes via their re-modelling, by comparing the results of the analysis with and without the simplification. ...
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The configurational approach enables understanding the behaviour of road-network systems in the face of sudden physical disruptions. Previous studies show that Space Syntax analysis can assist in evaluating how urban systems respond to punctual network interruptions, both in the short and medium-term, and help managing associated risks. The events which followed the crash of the Polcevera bridge in Genoa and that of Bologna Borgo Panigale bridge in 2018 demonstrated in practice that localised urban network interruptions can propagate, affecting movement dynamics, well beyond the boundary of a city and compromise the functioning of the regional motorway network. However, representing the associated effects across the urban to the regional network levels remains a challenge due to computational limitations which constrain Space Syntax studies to use simplified networks in their analyses. This in turn causes discrepancies in cross-scale comparisons, as urban and regional road-morphologies are represented at different levels of detail. The paper studies the effects of the two dramatic events from a multi-scale configurational standpoint by comparatively analysing the through-movement patterns of the urban road-, the regional primary-and the regional motorway-circulation systems. The goal of this research is to discuss, using a real-world example as a benchmark for assessment, the viability of adopting the configurational approach to study failure propagation and gauge levels of street network resilience across spatial scales. The results of this study clarify the importance of weak ties for the resilience of road infrastructure systems and further demonstrate the homothetic behaviour of Normalised Betweenness Centrality measures. ()*+,%-#' Betweenness Centrality, Bridge Disaster, Multi-scale Analysis, Space Syntax, Street Network Resilience
... Since long axial lines can be divided into smaller segments without affecting the analysis, it offers a more refined and robust approach. One of the most profound consequences is that the angular shortest path can be computed on the basis of the established and widely accessible representations of street networks such as a street-center line with only little adjustment necessary (Krenz, 2017). ...
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This dissertation investigates the interactions between urban form, allocation of activities, and pedestrian movement in the context of urban planning. The ability to assess the long-term impact of urban planning decisions on what people do and how they get there is of central importance, with various disciplines addressing this topic. This study focuses on approaches proposed by urban morphologists, urban economists, and transportation planners, each aiming the attention at a different part of the form-activity-movement interaction. Even though there is no doubt about the advantages of these highly focused approaches, it remains unclear what is the cost of ignoring the effect of some interactions while considering others. The general aim of this dissertation is to empirically test the validity of the individual models and quantify the impact of this isolationist approach on their precision and bias. For this purpose, we propose a joined form-activity-movement interaction model and conduct an empirical study in Weimar, Germany. We estimate how the urban form and activities affect movement as well as how movement and urban form affect activities. By estimating these effects in isolation and simultaneously, we assess the bias of the individual models. On the one hand, the empirical study results confirm the significance of all interactions suggested by the individual models. On the other hand, we were able to show that when these interactions are estimated in isolation, the resulting predictions are biased. To conclude, we do not question the knowledge brought by transportation planners, urban morphologists, and urban economists. However, we argue that it might be of little use on its own. We see the relevance of this study as being twofold. On the one hand, we proposed a novel methodological framework for the simultaneous estimation of the form-activity-movement interactions. On the other hand, we provide empirical evidence about the strengths and limitations of current approaches.
... To perform both analyses, RCL data is required. As natural disasters defy administrative boundaries and data needs to be strictly up to date, we overcome these issues by resorting to the Open Street Map (OSM) [36] VGI data repository, which presents a homogeneous and extensive mapping coverage of street networks at a global level [37], besides rich semantic tags. As performing a quality assessment of VGI data is out of the scope of this research, we assume the retrieved OSM data is accurate. ...
Urban planners and decision-makers need to provide, with increasing frequency, a rapid response to the many natural hazards looming over cities today, while relying on limited information and computational resources. This research explores viable ways to support them in developing performance-driven proposals via urban modelling by: (i) exploring potential synergies between configurational analysis methods, open computing platforms and collaborative geo-data; (ii) testing the interchangeability of different road network models in a set of suitable case studies. Getting fast, free and ubiquitous access to updated spatial information should allow decision-makers and experts to gauge relevant centrality and resilience properties of urban grids and bears opportunities for quantitative scenario testing beyond institutional boundaries. Therefore, this paper suggests linking knowledge and metrics from different street network analysis models and adapting analysis workflows to humanitarian needs to foster deliberation in strategic decision-making and tactical planning in urban disaster contexts. The results obtained in a set of suitable case studies indicate that, under certain conditions, time and computational resources can be saved, and interdisciplinarity boosted, by suitably interchanging primal and dual analysis models and tools. Nonetheless, further tests are needed to establish thresholds and generalize results.
... In the short term, the only sensible response is to develop protocols for the creation or editing of input maps. Some modelling rules have been tentatively proposed to refine the road centerline maps for conducting street network analysis, and their effects have been discussed, albeit in the context of angular segment analysis (Kolovou, Gil, Karimi, Law, & Versluis, 2017;Krenz, 2017). In the longer term, these protocols could be based on research regarding the way in which people interpret street geometries as they move in space and construct cognitive maps of the environment, as argued by Marcus (2018). ...
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By asking how much street length can be reached from a given origin within a specified distance limit, and by defining distance in different ways as a function of the physical or cognitive effort required to move in cities, the analysis of reach produces measures that effectively characterize street density, connectivity, and the associated urban potential. While the conceptual foundation for reach analysis has already been laid, the computational aspects have not been sufficiently addressed. We introduce the different graph representations and algorithms we developed to analyze metric reach, directional reach, and intersection reach—a new addition to the existing measures. The graph representation we developed for directional reach analysis also sets the foundation for more advanced graph-based street network analysis.We also provide formulae for computing the mean directional and intersection reach. Finally, we discuss common street network modeling issues that can be addressed by consistent mapping protocols.
Conference Paper
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Among the most recent directions of urban morphology research is its integration with disaster studies, in support of disaster risk reduction (DRR) efforts at the urban scale. Yet, the built and unbuilt components of urban form are still disproportionately investigated for DRR purposes, with predominant approaches centred on buildings leaving the DRR potential of the urban spatial network relatively under-investigated. This paper, at the intersection of urban morphology and disaster studies, is the first of a series looking at the spatial component of urban form through the lens of risk, with a focus on urban vulnerability to earthquakes. After discussing how the interplay of configuration, governance, and use of space impact urban disaster risk in earthquake-prone settlements, the paper introduces a method for the exposure assessment of urban spatial layout. The method, applied on the configurational analysis of four settlements hit by the 2016 Central Italy Earthquakes, associates disaster risk variables to the urban spatial network's core elements. It develops (i) a theoretical re-definition of the significant disaster risk variables in relation to configurational measures; and (ii) an integrated spatial analysis workflow for visualisation and classification of street segments and routes based on their degree of exposure, to inform both ordinary and emergency planning. In (i), the spatial-configurational dimensions of disaster concepts (hazard, exposure) are identified to unfold the spatial potential in DRR. In (ii), the spatial analysis workflow builds upon the recently developed applications of space syntax angular segment analysis on the OSM RCL network, by combining Volunteered Geographic Information (VGI) with land use and disaster-related datasets, to generate hybrid exposure segment maps within the ArcGIS environment. The paper provides a twofold contribution: recontextualization and incorporation of space syntax theoretical knowledge into DRR, and innovative employ of existing applications for a multidisciplinary and comprehensive approach to urban vulnerability assessment.
Accessibility is fundamentally thought to be related to functional, economic, and social performances of cities and geographical systems and, therefore, constitutes an essential aspect for spatial planning. Previous studies focused on cities or metropolitan scales, often disregarding their position within regional and national systems, which can greatly affect their performance. Although accessibility at various spatial scales has been examined, the studies focused on accessibility patterns at different scales, with no reference to the level of accessibility of cities over local, regional, and national scales simultaneously, i.e. multiscale accessibility. This study aims to elucidate the multiscale accessibility level of individual cities and examine its relationship to urban performance in the urban system of Israel. Spatial accessibility was analyzed using the space syntax methodology for the entire national road network across multiple geographic scales—from the local to the national scale. Based on three distinct spatial accessibility systems identified, a unique multiscale accessibility profile was created for individual cities in Israel. Subsequently, each city’s multiscale accessibility profiles were examined against urban performance indicators determined from urban scaling theory. We found that the superiority of cities characterized by high accessibility level plays a role not only for a specific scale but also over scales and spatial systems. Moreover, most urban performance indicators related to the multiscale accessibility profiles of cities, while some multiscale accessibility profiles can be related to over- or under-performance of cities. The findings suggest that pervasive accessibility across spatial scales is inherently connected to urban performance and may indicate on the implementation and interpretation of accessibility. These findings may assist in various aspects of spatial planning at various scales.
Accessibility is a well-known basic term in spatial science and planning and is inherently related to functional aspects of places and regions. Although previous studies have examined functional systems and spatial accessibility few have attended to the association among them across geographical scales. Our study attempts to fill this gap. Using the space syntax methodology, spatial accessibility was analyzed for the entire national road network of Israel across different geographic scales – from local culminating in the national scale. The analysis was based on angular segment analyses of the road center-line network. Following this, the correlation between spatial accessibility across scales and functional performance of employment and commuting flows was examined. The study findings show a significant correspondence and exposes transitions between local, regional and national spatio-functional systems. First, a significant correlation between local (2 km radius) accessibility levels of settlements with the number of employees and commuters. Second, the regional/metropolitan system (10–15 km radius) accessibility is highly correlated to the emergence of the main employment centers. Third, the main metropolitan areas are integrated at higher scale (from 30 km radius) and form together a core region characterized by high accessibility as well as well connectivity through commuting flows. In contrast, no substantial commuting flows were found within the periphery, as well as between periphery-core. The findings show clearly that this functional structure corresponds to the multi-scale accessibility levels of settlements. We conclude that the core region functions at multiple scales (local-regional-national) while the periphery functions mostly at a local scale.
Conference Paper
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In this paper we take a step towards extending space syntax analysis into the countrywide scale, through the study of three very-large spatial systems in the UK, namely the top-tier road network of the entire country (170,007 nodes), the complete road network (1,208,674 nodes) of three contiguous NUTS1 regions (the East of England, South East of England and Greater London) and the complete road network of UK's mainland (2,031,971 nodes). We compare the results of our analysis with several types of functional and socio-economic data, finding clear statistical associations at the scale of the entire country between network structure, vehicular movement flows and the spatial distribution of several socio-economic variables. We conclude by arguing that space syntax models and analysis hold their value at very-large territorial scales, being highly robust and producing coherent results between datasets of different sources, themes and dimensionalities.
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With the ubiquity of advanced web technologies and location-sensing hand held devices, citizens regardless of their knowledge or expertise, are able to produce spatial information. This phenomenon is known as volunteered geographic information (VGI). During the past decade VGI has been used as a data source supporting a wide range of services, such as environmental monitoring, events reporting, human movement analysis, disaster management, etc. However, these volunteer-contributed data also come with varying quality. Reasons for this are: data is produced by heterogeneous contributors, using various technologies and tools, having different level of details and precision, serving heterogeneous purposes, and a lack of gatekeepers. Crowd-sourcing, social, and geographic approaches have been proposed and later followed to develop appropriate methods to assess the quality measures and indicators of VGI. In this article, we review various quality measures and indicators for selected types of VGI and existing quality assessment methods. As an outcome, the article presents a classification of VGI with current methods utilized to assess the quality of selected types of VGI. Through these findings, we introduce data mining as an additional approach for quality handling in VGI.
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Mit OpenRouteService (ORS) liegen zur AGIT 2010 schon über 2 Jahre an Erfahrungen mit dem Betrieb von Online-Diensten auf Basis der Daten der freien Wiki-Weltkarte OpenStreetMap (OSM) vor. Dabei bietet ORS mittlerweile natürlich sowohl eine größere Funktionsfülle als auch räumliche Abdeckung. Von knapp 400.000 Straßensegmenten für Deutschland in der ersten Version wuchs der Service auf nun weit über 20 Millionen Straßensegmente. Dies impliziert natürlich besondere technische Herausforderungen, die im Beitrag dargelegt werden. Auch hat Geburtstag und einher geht mit beiden die wachsende Erfahrung mit dem Preprocessing dieser Daten für unterschiedliche Anwendungsgebiete. Der Schwerpunkt der Untersuchungen bezieht sich auf die "fitness for use" der verwendeten Grundlagendaten und deren räumliche, thematische und zeitliche Differenzierung. Dabei kann mittlerweile auf Erfahrungen mit mehreren unterschiedlichen Routingspezialanwendungen zurückgegriffen werden. Neben PWK (schnellste, kürzeste), Fußgänger und Radfahrer (unterschieden nach MTB, Rennrad, kürzeste und sicherste) sind dies u. a. Rollstuhlrouting, 3D-Routing, Notfall-Routing und Routing für landwirtschaftliche Maschinen (Mähdrescher, Erntezüge) sowie Evakuierungssimulationen. Neben dem eigentlichen Routenplaner und seinen Derivaten wurden am Lehrstuhl eine Vielzahl weiterer OGC- und weiterer GI-Dienste realisiert, die jeweils eigene Ansprüche an die Daten haben. Diese und wesentliche Erfahrungen mit dem Online-Betrieb dieser Dienste werden am Beispiel empirischer Untersuchungen der Daten diskutiert. Zu nennen sind u.a. folgende: Web Feature Service (OGC); Geocoder & Reverse-Geocoder (gemäß OGC OpenLS Standard); POI-Umkreissuche (gemäß OGC OpenLS Standard, Directory Service); Erreichbarkeitsdienst (Accessibility Analysis Service); Web -3D Service (;; (Historische und archäologische Daten in OSM); Höhenprofilberechnung als OGC Web Processing Service (WPS) - in ORS; Testdaten für diverse WPS-Prozesse auf
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The meaning and purposes of web has been changing and evolving day by day. Web 2. 0 encouraged more contribution by the end users. This movement provided revolutionary methods of sharing and computing data by crowdsourcing such as OpenStreetmap, also called "the wikification of maps" by some researchers. When crowdsourcing collects huge data with help of general public with varying level of mapping experience, the focus of researcher should be on analysing the data rather than collecting it. Researchers have assessed the quality of OpenStreetMap data by comparing it with proprietary data or data of governmental map agencies. This study reviews the research work for assessment of Open- StreetMap Data and also discusses about the future directions.
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GIS databases representing urban layouts according to road centerlines spanningbetween intersection nodes (at road junctions) can be analyzed syntactically basedon the concept of angular fractional depth.
Conference Paper
Convex maps were first introduced by Hillier and Hanson in “The social logic of space”, and have since become a standard diagram of space syntax, particularly in the analysis of interior of buildings. Despite of their extensive use, the computer generation of convex maps turns out to be quite difficult. An algorithmic description of the process would afford an objective form of reproducing analysis results and the consistent application of the same method on a set of data; unfortunately the original guidelines to draw convex maps by hand have been shown to be impossible to translate into a formal description of the type necessary for a computer program. Thus, rather than attempting a translation of the original procedure, we have looked at alternatives methods for producing convex maps in the field of shape analysis. In particular we have studied a set of problems which deal with the decomposition of shapes into simpler parts and their organisation,and which are conceptually related to the convex map. We have accordingly developed a method for subdividing architectural plans into non overlapping, convex partitions that captures their most salient organisational features, based on the medial axis transform, a well known shape descriptor first proposed by H.Blum in 1967. Our method is based on adding the simpler convex regions defined through the segments and branching points of the medial axis according to different priorities, under the condition that these additions remain convex. In space syntax the automatic production of convex spaces has often focused on their instrumentality in the calculation of axial maps, and has not have the convex map as the final objective.The method we have developed, in the other hand, produces convincing convex partitions and maps, which often coincide with those resulting from following the original hand-drown method description. Its results can be used for representing the organisation of spaces at the level of simplicity and abstraction of the original convex partitions proposed by Hillier and Hanson, and to allow the study of their configuration through the application of different graph measures and visualisation techniques. We have implemented our methods in C++. The effective calculation of the medial axis required the use of robust and reliable computational geometry libraries, and consequently we have based most of our geometric data-structures and algorithms in those provided by CGAL, a well tested and widely used library distributed under GPL/LGPL license. Besides CGAL our tests and demonstration programs also use a number of different libraries, such as Dime, for dxf input and output or Qt, for GUI and interaction.
Conference Paper
Abstract This paper ,deals with the impact ,of different map ,scale and ,various ,morphological features on the,configurative representation,of cities with axial line maps. It is explored why,in two ,different cities the change ,in scale ,and ,resolution result in very ,different patterns of integration. It is argued,that the underlying,morphological,properties of the cities are the main ,factors for the divergence ,and ,that differences ,in scale ,or the process,of generating axial line maps,have,a minor impact. Axial line maps,of the cities of Hamburg and Stuttgart are compared,for both cities they differ in level of detail and in scale (1:2.500 to 1:25.000). For Hamburg, different integration patterns evolve, for Stuttgart, a persistent centre of integration remains. A validation with traffic data shows in no case a preference for one of the types of axial line maps. Based on these findings, two location related measures ,are proposed ,for estimating ,the degree ,of differences between,divergent,axial line maps. The “rank correlation measure”,and the “step depth
The book presents a new theory of space: how and why it is a vital component of how societies work. The theory is developed on the basis of a new way of describing and analysing the kinds of spatial patterns produced by buildings and towns. The methods are explained so that anyone interested in how towns or buildings are structured and how they work can make use of them. The book also presents a new theory of societies and spatial systems, and what it is about different types of society that leads them to adopt fundamentally different spatial forms. From this general theory, the outline of a 'pathology of modern urbanism' in today's social context is developed.