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CAST is actively exploring CityGML as a standardized data format for describing urban objects. The CityGML format extends Geography Markup Language (GML), which describes mainly the geometry of urban objects. In addition to such geometrical descriptions, CityGML also provides a variety of semantic tags characterizing the nature of the object being described. Such tags allow us to specify whether a rectangle, described geometrically by the coordinates of its vertices, represents a roof surface, wall surface, floor surface, window or door (just to name a few of the possible semantic tags that might be associated with a rectangle).

Further, the CityGML standard specifies five different levels of detail (LOD0 - LOD4) which may be chosen depending on the use intended for a particular model. For determining buildings' positions on a 2.5-dimensional terrain map, LOD0, in which only such positions are specified, is sufficient. But to specify, for example, the exact location of electrical conduits within the walls of a building, we would need the detail of LOD4, which describes the rooms within a building and can include even the furniture inside those rooms.

These characteristics of CityGML make it a useful intermediary for exchanging descriptions of urban infrastructure between diverse applications. CAST is evaluating the tools available for enabling the exchange of data between applications as well as for storing and selectively retrieving CityGML data. On the one hand, software is currently being developed to import commonly used architectural visualizations (SketchUp, AutoCAD, etc.) into CityGML and to export CityGML files into visualization and editing progams. And, on the other hand, we are examining tools for importing multiple CityGML files into a single 3D spatial database so as to query masses of information about urban objects according to spatial criteria as well as other identifying features.