3D city models are digital models of urban areas that represent terrain surfaces, sites, buildings, vegetation, infrastructure and landscape elements as well as related objects (e.g., city furniture) belonging to urban areas. Their components are described and represented by corresponding two-dimensional and three-dimensional spatial data and geo-referenced data. 3D city models support presentation, exploration, analysis, and management tasks in a large number of different application domains. In particular, 3D city models allow "for visually integrating heterogeneous geoinformation within a single framework and, therefore, create and manage complex urban information spaces."
To store 3D city models, both file-based and database approaches are used. There is no single, unique representation schema due to the heterogeneity and diversity of 3d city model contents.
Components of 3D city models are encoded by common file and exchange formats for 2D raster-based GIS data (e.g., GeoTIFF), 2D vector-based GIS data (e.g., ), 3D models (e.g., .3DS, .OBJ), and 3D scenes (e.g., Collada, Keyhole Markup Language) such as supported by CAD, GIS, and computer graphics tools and systems. All components of a 3D city model have to be transformed into a common geographic coordinate system.
A database for 3D city models stores its components in a hierarchically structured, multi-scale way, which allows for a stable and reliable data management and facilitates complex GIS modeling and analysis tasks. For example, the 3D City Database is a free 3D geo database to store, represent, and manage virtual 3D city models on top of a standard spatial relational database. A database is required if 3D city models have to be continuously managed. 3D city model databases form a key element in 3D spatial data infrastructures that require support for storing, managing, maintenance, and distribution of 3D city model contents. Their implementation requires support of a multitude of formats (e.g., based on FME multi formats). As common application, geodata download portals can be set up for 3D city model contents (e.g., virtualcityWarehouse).
The Open Geospatial Consortium (OGC) defines an explicit XML-based exchange format for 3D city models, CityGML, which supports not only geometric descriptions of 3D city model components but also the specification of semantics and topology information.
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