CityGML is a common information model for the representation of sets of urban objects in 3D. It defines the classes and relationships for the most relevant topographic objects in city models with regard to their geometric, topological, semantic and appearance properties.

It includes generalization hierarchies between thematic classes, aggregations, relationships between objects and spatial properties. This thematic information goes beyond graphic exchange formats and makes it possible to use 3D models of virtual cities for sophisticated analysis tasks in different applications such as simulations, urban data mining, management of buildings and their services or emergencies.

CityGML is an open data model and XML-based format for data storage and exchange. It is implemented as an application scheme for Geography Markup Language 3 (GML3), the international standard issued by the Open Geospatial Consortium (OGC) and ISO TC211. CityGML aims to become an open standard and can therefore be used freely.

The OGC adopted version 1.0.0 of CityGML as an official OGC standard in August 2008.[1] In April 2012, version 2.0.0 was released.

The main features of CityGML are the following:

• - It is a geospatial information model for urban landscapes based on the ISO 191xx family.

• - Makes GML3 representations of 3D geometries based on the ISO 19107 model.

• - Allows the representation of the surface characteristics of the object (textures, materials).

• - It has the following groups of taxonomies and aggregations:.

• - CityGML models have five types of levels of detail (LOD):

• - Allows several representations at different levels of detail simultaneously.

• - Optional topological connections between (sub)metric objects.

Because CityGML is based on GML3 it blends seamlessly with the full range of other OGC standards. Thus, the WFS, CS-W, WCTS and WPS services are especially relevant to access, process and identify CityGML resources. For 3D visualization, CityGML should be considered as a base format from which 3D graphics formats can be easily obtained.

• - Geography Markup Language.

• - OGC CityGML standard page.

• - German contribution to CityGML Archived February 1, 2013 at the Wayback Machine.

• - CityGML Wiki.

• - Free viewers and tools for CityGML:

• LandXplorer-CityGML-Tool

• Aristotle

• 3DGIS cityvu

• libcitygml C++ library & citygml2vrml converter.