Semantic Modelsfor Architectural Heritage Documentation

Erik Costamagna and Antonia Spanò

Politecnico di Torino, viale Mattioli 39, 10125 Torino Italy{erik.costamagna,antonia.spano}@polito.it

Abstract. In the field of CH metric documentation management thedevelopment of GIS tools has radically improved the capability of han-dling complex geometric models and the quantity of the semantic valuesof spatial data. These improvements in GIS tools have been followed bythe development of data models and data definition languages able tomanage such a complexity through a set of open rules and vocabularies.We need to change the application-driven practice of the GIS to a com-mon set of rules and frameworks through the adoption of open-standardsand languages. The aim of this study is to showcase the results of thetest of framework for the management of a 3D metric survey archive bymeans of CityGML standard.

Keywords: “3D GIS”, “semantic models”, “CityGML”, “CH metricdocumentation”.

1 Introduction: Standards and Tools for CH MetricDocumentation

1.1 Issues in CH Metric Documentation

The development of ICT has radically changed both the acquisition and man-agement of metric data. In the field of GIS (Geographic Information System)the database tools, initially developed for the automatic management of 2D rep-resentation in digital cartography, have been successfully applied to geographicinformation in order to perform advanced spatial and thematic analysis. Theobject-oriented modelling languages, initially developed as programming tools,have been applied from the late ’90s also to the Data Base Management Sys-tems [1] (DBMSs) leading to the adoption of the Unified Modeling Language(UML) as a standard for conceptual data modelling. In the field of web toolsthe development of the eXtensible Markup Language [2] (XML) and its exten-sions XQuery [3] and XPath [4], the fundamental elements of semantic web,established this technology as the most widely used standard for representingstructured information. The flexibility of these tools is the major reason for theadoption of the XML technology in order to represent complex semantic con-tent such as Cultural Heritage (CH) documentation. Indeed it has been adopted

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as a standard for several national and international institutions. Some exam-ples are the Sistema Informativo Generale del Catalogo (SIGEC) card cataloguestandards by the Italian Istituto Centrale per il Catalogo e la Documentazione(ICCD) and the semantic model Conceptual Reference Model (CIDOC) devel-oped by the International Council of Museums (ICOM). The wide reach achievedby these languages has also allowed the birth of specific XML based languagesfor the management of 2D and 3D representations like Geographic Markup Lan-guage (GML). These tools match the need for open standards and languagesfor spatial information management and can overcome the limits of regulationsfragmentation and software application dependency [5].

1.2 Geographical Information Languages and Standards

In the survey of normative international institutions, the Open Geospatial Con-sortium (OGC) is one of the major sources of open models and languages forencoding spatial information. OGC standards, like GML, have been adopted bymost mapping agencies and they are starting to be supported by GIS software,which are the prerequisites for them to be widely used among the specialists inthis field. The main strong points of OGC standards are the completeness ofdata definition, from the conceptual level to the logical and physical implemen-tation, and the fact that they are ‘open’ which allows the scientific communityto extend the language domain to embrace specific fields of application. Some ofthe most widespread OGC standards are the Keyhole Markup Language (KML),GML and the City Geographic Markup Language (CityGML). All of them arebased on the XML language and in this study we will focus on the latter twostandards as they fully support the 3rd dimension.

GML. GML language [6] is one of the most comprehensive and versatile tools ingeographical data modelling. Under development since 2005, it has now reachedversion 3.3 and it is one of the most outstanding languages for spatial data.The conceptual model is based on the formalisation of ISO/TC211 GeographicInformation, which has been logically implemented with the XML technologytools. Some of the ISO/TC211 standards included in GML are the ISO 19107(Spatial Schema), ISO 19123 (Schema for Coverage Geometry and Functions)and ISO 19108 (Temporal Schema). OGC submitted the standard to ISO whichpublished it as ISO 19136 in 2007. This language allows the modelling of themajority of 2D and 3D vector representations as well as raster representations,and also includes specific tools for the modelling of temporal phenomena. Theconceptual basis of the model, associated with the levels of completeness andflexibility of the XML technology, defines this language as a common grammarwith which to build upon extensions for specific domains such as urban datamanagement.

CityGML. CityGML standard [7] has been developed from the SIG3D (SpecialInterest Group 3D) research group, within the Geodata Infrastructure North-Rhine Westphalia initiative. The goal of this standard was to develop a specific

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tool for modelling thematic data from urban cartography within the GML archi-tecture. Indeed, it is actually an extension of the GML language. The CityGMLschema is modular, comprising a mixed geometrical-topological spatial moduleand several thematic modules corresponding to the urban map layers. Comparedto the GML language, and in addition to the semantic specialization, CityGMLhas developed the multiscale and multiview support. The CityGML approach tothe multiscale modelling consist in a method of separating features founded onthe relationship between the geometric and thematic component. For each LODthere is a specification for the generalization level, the absolute tolerance and theincluded thematic classes. The model allows to associate more than one instanceof a geometric class to each instance of a thematic class through a specific relationfor each LOD. In addition, the multiview support associates a set of visualiza-tion settings, including the photo-realistic rendering of the surface of geometricmodels, to each instance of a thematic class. A specific module (Appearance)allows to manage the visualisation independently of the implementation of thegeometric and thematic classes, thus each instance of a geometric-thematic classcan share different visualisation settings (themes).

2 Model Implementation

2.1 Case Study Features and Test Role in the Scenery of GeospatialModels

The Valentino Castle is a XVIIth century mansion located in the urban fabric ofTorino and is one of the Residences of the Royal House of Savoia, included in theUNESCO World Heritage List since 1997. The building stands on the westernriverbank of the Po river, in the southern part of the Roman city. The first projectof the Castle, endorsed by the regent Maria Cristina at the beginning of the XVI-Ith century, and developed by Carlo Castellamonte, was intended to overlook theriver at the point where the main entrance was planned. In the second half ofthe century the project of Amedeo di Castellamonte, Carlo’s son, changed theoriginal plan expanding the building in the direction of the new enlargementof the city towards the south and cementing its connection with the contempo-rary urban fabric. In the late eighteenth century the Valentino Castle became theseat of the Royal School of Application for Engineers, later Politecnico di Torino,leading to the last change in its appearance. Some of the features of this building,like its spatial complexity, the layering of the constructive phases, its relation-ship with the urban fabric and its present function as university, illustrate themain issues of the management of architectural heritage in an urban context. Forthese reasons it can be taken as a typical example of the issues in documentingarchitectural heritage in an urban context. This experience seeks to demonstratethe feasibility of implementing 3D metric data archives pertaining architecturalheritage, yielded by high detail surveys intending to identify the architecturalfeatures of building complexes, within those systems currently identified as 3Dcity models and following their paradigms in geometric and semantic organiza-tion. And precisely for this reason we employed tried methods for classifying

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the building’s components, spread throughout cultural heritage documentation,starting from the ones used by preservation institutions and already referencedin Sect 1.1.

2.2 Data Restructuring

The Valentino Castle was involved in a series of survey campaigns between 2007and 2009. They were part of the preliminary step for the restoration of the build-ing’s façades, part of which has already been completed. These survey campaignshave been carried out by our group, which specializes in image-based surveymethods and other geomatic techniques applied to CH [8] The survey involvedthe exterior façades, the courtyard and part of the interiors and the surveyproducts, consisting of high detailed georeferenced raster and vector represen-tations, were obtained from photogrammetrical, topographic, TLS (TerrestrialLaser Scanner) and GNSS data capture. The different types of raw metric datawere processed in order to extract 2D vertical and plan cross-sections and ortho-projections. This was the base dataset, corresponding with the most widespreadtypologies of metric documentation in the CH domain, from which 3D modelsat different LODs were built. The qualitative feature of raster representationslike rectified images and ortho-photos, which are essential tools to understandthe cultural value of the object, were combined with the 3D vector models. Themetric data integration was performed through CAD applications whereas thetranslation to the CityGML model classes was performed through the graphictool of the Safe Softwares FME Workbench. Additional thematic data integra-tion (the Appearance module classes) and the data validation (consistency andnamespaces declaration) was performed by means of XML editors.

Multiscale Model. Multiscale modeling is an approach to the understandingof spatial information that requires the ability of discreetly separating geometricfeatures in predefined levels of detail (LODs) based on their semantic values. Themorphological, functional and construction features are the main categories forthe comprehension of architectural objects. On the other hand the orders of theclassic treaties are a key to a critical explanation of the historical architectureand a vocabulary to encode the architectural elements and their relationships.In addition to these semantic categories other aspects must be considered likethe displaying of the geometry and related features such as data processing [9].Architectural heritage consists in a complex structure of relationships betweengeometric and thematic information, which are evident when switching from theurban scale to larger scales. A multidisciplinary approach in building featureclassifications must take into account both the coded languages in the architec-ture criticism and the data management issues [10].

The CityGML model supports 5 LODs, each one of them comprising specificrelations between thematic and geometric classes. The LOD definition is logi-cally formalized in a way as to be independent from the application tools. The1:50 representation scale, which corresponds with the most widespread in CHmetric documentation, was employed as the higher scale, and a specific LOD was

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Fig. 1. Workflow of discrete LODs modelling process

Fig. 2. Overlapping ofLODs showing differentparts of the building imple-mented for each LOD: fromtop to bottom LOD1-2,LOD2-3, LOD3-4

assigned to it, integrating the sequence identified by the OGC for CityGML andwhich was developed primarily to describe the features of architectural façadesof historical buildings and which must be kept distinct from the building fabriclevels for which the reference model provides for a less complex and higher scaleorganisation. Based on the CityGML specifications the correspondence betweenthe LODs and the representation scales was defined as follows: LOD4 (1:100);LOD3 (1:200-1:500); LOD2(1:500-1:1000) LOD1 (1:1000-1:2000); LOD0 (1:5000-1:10000). For this reason, a master model of an architectural detail at this scalewas created in order to derive the lower CityGML LODs of a wider part of thebuilding using the master model as a base for the generalization of the repetitiveelements of the architectural decorations. From the higher scale (1:50) we de-rived the lower LODs as far as LOD1, including all the architectural and urbanrepresentation scales. A model of the entire castle was implemented at LOD1and 2, while only portions of the building were modelled at higher levels, specifi-cally in order to perform the test. A façade of the south-eastern tower, including

246 E. Costamagna and A. Spanò

Fig. 3. The two Appearance themes (LOD2-4) implemented: before restoration (top)and after (bottom)

the pavilion roof, was implemented at LOD3 whereas only the first two floorsof the façade were modelled at LOD4. The 3D master model of the detail of awindow were matched in order to create the LOD4 and LOD3 models. At theLOD2 the specifications don’t provide a specialization of the façade features andso this model was implemented separately and used to derive the lower detailmodel (LOD1).

Multiview Model. Multiview modelling defines the capacity of a system todescribe the surface aspect of geometric objects in different ways correspond-ing to specific point of views or domains of interest through a parameterizationof these features. Some of them are intrinsic to the material feature like colour,transparency and texture while others like illumination, reflection and refractionare environment-related. The management of multiview modelling is a typicalfeature of CAD systems, including tools for material and environmental param-eterization and photo-realistic rendering. These are some of the CAD functionsadded to GIS for the management of 3D architectural models [11]. The CADsystems, developed initially for mechanical design, suddenly broaden out intothe Architecture Engineering Construction (AEC) field. Essentially conceivedfor geometric information management, these systems, due to the widening oftheir application field, began to acquire the ability to manage semantic featuresof buildings with database tools. The Building Information Model [12] (BIM) isthe currently emerging paradigm representing this new approach. Unlike CADsystems, the CityGML model provides different classes for materials and texturemodelling. The materials are defined through eXtensible 3D [13] (X3D) speci-fication tools of Web3D Consortium, while the texture classes allows the man-agement of different kinds representations like homographic and orthographic

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Fig. 4. Structure of the semantic model. In the first enlargement are shown the Build-ing attributes: class=1000 and function=1000 stands for the habitation original in-tended use of the pleasure residence of the Dukes, while usage=2100 represent theactual university function. In the second enlargement the attributes of a BuildingPart :yearOfConstruction=1620, roofType=1070 (pavilion roof) and the number of storeys.

projections, texture parameterization and georeferenced images. The raster rep-resentations are of great importance in CH metric documentation since they areused to highlight qualitative features of objects like the preservation state andthe construction material. They are also useful to record the state before andafter a restoration intervention in order to manage the building’s lifecycle. Withthis purpose new photograms of the southern façade were taken and the recti-fied images of the previous and subsequent state were implemented in differentAppearance themes.

Thematic Model. The Building Module is the most complex scheme in theCityGML specifications and the most comprehensive in terms of semantic andgeometric definition including the interiors and exteriors of a building. The firstsemantic specialization is represented by the BuildingPart class associate withthe Building class by means of the consistOfBuildingPart aggregation relationallowing recursive aggregation between them. The Building and BuildingPartclass features are defined through several attributes, representative of the main

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Fig. 5. The decorative superstructure en-lighted for each LOD, from left to right LOD3,LOD4 and 1:50

Fig. 6. The same decorative detailat different scales: from left to right1:50, LOD4, LOD3

Fig. 7. A proposal for an extension of the CityGML building module: the UML diagamextract shows the classification which can build into the classical order filing. The ab-stract class ch::_DecorativeSuperstructure is a descendant of the gml:Abstract_Feature.The boundedBy aggregation relation between this class and the BoundarySurface andOpenings CityGML classes allows a separate definition for façade and openings dec-orative elements. The lod3MultiSurface and lod4MultiSurface relations complete thediagram with the geometry content.

characteristics of the structure. Fig. 4 shows an example of the values of theseattributes. The plan structure of the Valentino Castle, derived by the Frenchroyal palace style, consisting in a series of differently shaped building parts sur-rounding a courtyard, is the material representation of the absolute power ofmonarchy and this feature is the most representative of the whole building. Toformalize the relationships between the building parts and the whole complexthe BuildingPart to Building association was modelled as a composite relation.So all the parts of the castle were implemented at LOD1 as instances of theBuildingPart class while the Building instance is defined by the aggregation ofits parts. Starting form LOD2 the specialization of the building parts allows todefine the different elements of the building envelope such as the GroundSurface,RoofSurface and WallSurface classes, associated with the Building class through

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the boundedBy relation. The LOD3 allows to model the openings of the façadewhile the LOD4 allows to represent the interiors.

In spite of their comprehensiveness in representing architectural features,CityGML thematic classes don’t fulfil the semantic specialization of the his-torical buildings. In this study we focused on the decorative elements of thefaçade, disregarding structural elements such as columns and vaults forming ar-cades, a common feature of European and Mediterranean cities. These kind ofelements indeed are classes of the IFC (Industry Foundation Classes) semanticmodel that cannot be translated into CityGML classes as this model follows asurvey-based description approach, representing only the boundary surfaces ofobjects [14,15].

The decorative elements which are evident from the LOD3 should be mod-elled with proper class specializations of the BoundarySurface classes, a first stepin managing cultural heritage features for different purposes, like maintenanceplanning, restoration projects or simply recording the cultural values. The classi-fication of the classical order was developed in tight relation with the structuralelements, but were later used only as a grammar element for the modelling ofbuilding shapes, independently from the structural or decorative function of theelements. This kind of archival filing, a necessary tool to understand the architec-tural language, should be related to the approaches which are well establishedin the CAD and GIS systems. Fig. 7 shows these elements in relation to thecorresponding LODs and a proposal for classification and model extension.

3 Discussion

This study shows the capacity of the OGC standard to manage most of the build-ing information features and to support the 3D geometric component. It allowsto encode the majority of the vector representations supported by CAD and GISsoftware. The raster representations are treated otherwise as a characterizationof the vector model, enhancing its significance. Moreover, XML language allowsto extend the data model and its related tools like the query language to fulfilspecific application domains. XML technology allows to automatically translatea data model from the conceptual level of the UML diagrams into the logicallevel of the XML Schema and vice versa, thus preserving the semantic corre-spondence. XML extensions like XQuery and XPath are powerful tools for thedata retrieval form XML documents. The geometric content handling throughthese systems is a new and open research issue. Basically it involves the XQueryand XPath languages extension and the provision of Application ProgrammingInterfaces (APIs) ie the specific application tools.

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