More and more applications need comprehensive information about the shape and the meaning of urban and landscape structures. Whereas in the past city models often have been built as purely graphical 3D models, new applications have information needs beyond visual characteristics. Besides geometry, semantics and topology of the 3D objects have to be taken into account in order to enable for thematic queries, analysis tasks, automatic integration, validity checking, or spatial data mining. Since the limited reusability of models inhibits the broader use of 3D city models, a more general modelling approach has to be taken in order to satisfy the information needs of the various application fields.
CityGML is a common information model for the representation of 3D urban objects. It defines the classes and relations for the most relevant topographic objects in cities and regional models with respect to their geometrical, topological, semantical, and appearance properties. “City” is broadly defined to comprise not just built structures, but also elevation, vegetation, water bodies, “city furniture”, and more. Included are generalization hierarchies between thematic classes, aggregations, relations between objects, and spatial properties. These thematic information go beyond graphic exchange formats and allow to employ virtual 3D city models for sophisticated analysis tasks in different application domains like simulations, urban data mining, facility management, and thematic inquiries. For specific domain areas, CityGML also provides an extension mechanism to enrich the data with identifiable features under preservation of semantic interoperability. The rich and general purpose information model provided by CityGML is especially important with respect to the cost-effective sustainable maintenance of 3D city models, allowing for the possibility of selling the same data to customers from different application fields.
CityGML is applicable for large areas and small regions and can represent the terrain and 3D objects in different levels of detail simultaneously. Since either simple, single-scale models without topology and few semantics or very complex multi-scale models with full topology and fine-grained semantical differentiations can be represented, CityGML enables lossless information exchange between different GI systems and users.
CityGML is implemented as an XML application schema for the Geography Markup Language version 3.1.1 (GML3). GML3 is the extensible international standard for spatial data exchange issued by the Open Geospatial Consortium (OGC) and the ISO TC211. CityGML has been adopted as official international standard by the OGC (as an approved GML3 application schema), enabling easy and free access to all the international community.
Features of CityGML
- Geospatial information model (ontology) for urban landscapes based on the ISO 191xx family
- GML3 representation of 3D geometries, based on the ISO 19107 model
- Representation of object surface characteristics (textures, materials)
- Taxonomies and aggregations
- Digital Terrain Models as a combination of (including nested) triangulated irregular networks (TINs), regular rasters, break and skeleton lines, mass points
- Sites (currently buildings; bridges and tunnels in the future)
- Vegetation (areas, volumes and solitary objects with vegetation classification)
- Water bodies (volumes, surfaces)
- Transportation facilities (both graph structures and 3D surface data)
- City furniture
- Generic city objects and attributes
- User-definable (recursive) grouping
- Multiscale model with 5 well-defined consecutive Levels of Detail (LOD)
- LOD 0 – regional, landscape
- LOD 1 – city, region
- LOD 2 – city districts, projects
- LOD 3 – architectural models (outside), landmarks
- LOD 4 – architectural models (interior)
- Multiple representations in different LODs simultaneously; generalisation relations between objects in different LODs
- Optional topological connections between feature (sub)geometries
- Application Domain Extensions (ADE): Specific “hooks” in the CityGML schema allow to define application specific extensions, for example for noise pollution simulation, or to augment CityGML by properties of the new National Building Information Model Standard (NBIMS) in the U.S.
CityGML and OGC Web Services
Since CityGML is based on GML3 it perfectly combines with the full range of other OGC standards. The Web Feature Service (WFS), the Catalog Service (CS-W), the Web Coordinate Transformation Service (WCTS), and the Web Processing Service (WPS) are especially relevant to access, process, and identify CityGML resources.
For 3D visualization, CityGML should be considered a base format from which 3D graphic formats can be easily derived. The rich semantic information of CityGML objects also help in automatic cartographic generalization and symbolization as well as in a styling process generating computer graphics represented e.g. as a KML/COLLADA or X3D files. Corresponding OGC portrayaling services are the Web 3D Service (W3DS) and the Web Terrain Service (WTS).
- CityGML has been developed since 2002 by the members of the Special Interest Group 3D (SIG 3D) of the initiative Geodata Infrastructure North-Rhine Westphalia (GDI NRW) in Germany. Since beginning of 2010 the SIG 3D has moved to Geodateninfrastruktur in Deutschland (GDI-DE). The SIG 3D is an open group with international members from more than 70 companies, municipalities, and research institutions working on the development and commercial exploitation of interoperable 3D models and geovisualization.
- Since 2004 CityGML is being discussed on an international level both within the European Spatial Data Research organization (EuroSDR) and the 3D Information Management (3DIM) Working Group of the OGC. By the beginning of 2006, a CityGML project within EuroSDR has been started aiming at the European harmonisation of 3D city modelling. From June to December 2006, CityGML was employed and evaluated in the CAD/GIS/BIM thread of the OGC Web Services Testbed Phase 4 (OWS-4).
- On September 27, 2006 the OGC Technical Committee approved the CityGML Specification version 0.3.0 as an OGC Discussion Paper at its meeting in Edinburgh.
- On July 12, 2007 the OGC Technical Committee approved the CityGML Specification version 0.4.0 as an OGC Best Practices Paper at its meeting in Paris. Therefore, CityGML is an official position of the OGC membership on 3D city modelling. The Specification document (OGC Document No. 07-062, PDF 9.8MB) can be freely downloaded from the Best Practices Paper section of the Open Geospatial Consortium website. A Change Document tracking the changes from the previous CityGML version 0.3.0 (OGC Document No. 06-057r1, PDF 3.1MB) to CityGML version 0.4.0 can be obtained here (PDF 4MB).
- On August 20, 2008 the members of the OGC have adopted version 1.0.0 of CityGML as official OGC Standard. The Standard document (OGC Document No. 08-007r1, PDF 9.8MB) can be obtained from the CityGML section of the OGC web presence.
- On April 4, 2012 version 2.0.0 of CityGML was adopted as official OGC Standard. The specification and schema files can be found here.
- CityGML Homepage: http://www.citygml.org (includes test datasets and software tools)
- Open Geospatial Consortium (OGC): http://www.opengeospatial.org
- OGC 3DIM Working Group: http://www.opengeospatial.org/projects/groups/3dimwg
- SIG 3D - GDI-DE Geodateninfrastruktur Deutschland (in German only): www.sig3d.org/