Virtual 3D city models are employed by municipalities, companies, enterprises, and mapping/ cadastre agencies in an increasing number of application areas. It can be foreseen that 3D geoinformation will become a natural part of the geo base data on a regional, national, and European level in the future. In order to allow for a lossless information exchange, a proper standard for virtual 3D city models is needed. However, no European or international standard for virtual 3D city and regional models was available in the past.
The development on CityGML was started in 2002 by the initiative Geodata Infrastructure North-Rhine Westphalia (GDI NRW) in Germany. From 2002 until 2005 the Special Interest Group 3D (SIG 3D), one of the working groups within GDI NRW, worked on the develop-ment of a standard for the representation and exchange of virtual 3D city and region models called “CityGML”. The underlying data model was (and still is) based on ISO 191xx stan-dards and was mapped to the Geography Markup Language version 3.1, the XML based ex-change format of the Open Geospatial Consortium (OGC). By the time of the beginning of the EuroSDR CityGML project in 2005, CityGML had been already implemented within diverse (German) 3D GIS systems, and it was evaluated in five pilot projects in cooperation with dif-ferent municipalities (among which were Berlin, Hamburg, Düsseldorf, and Cologne).
By 2005, CityGML represented the modelling consensus of over 70 institutions, companies, municipalities, and NMCAs. However, since it has been mainly developed within Germany, European and international aspects were not particularly addressed and the specification document was only available in German language. It soon became clear that in the context of ongoing European geospatial data harmonization efforts and the upcoming INSPIRE initiative it would not make sense to only concern about national modelling interests and requirements but instead address (at least) the European level. This was the moment when the CityGML project was brought to the attention of EuroSDR.
The EuroSDR CityGML project started in December 2005 and was carried out until August 2006 by Thomas H. Kolbe, Angela Czerwinski, Gerhard Gröger, and Lutz Plümer at the Insti-tute for Cartography and Geoinformation at the University of Bonn. The aim of the project was to bring the specification and modelling strategies of CityGML to a broad and general discussion within EuroSDR. It was intended to identify the needs and potential applications of EuroSDR members regarding the modelling, representation, quality and exchange of virtual 3D city models within Europe. Amongst other indicators, the joint ISPRS / EuroSDR / DGPF workshop on Next Generation 3D City Models in Bonn in June 2005 showed that at that point in time, European research institutions, NMCAs, and municipalities had been working at the leading edge of building and maintaining 3D city models (Kolbe & Gröger, 2006). This increasing momentum showed the potential to drive the development of a European standard, with potential international usefulness.
The long-term goal of the project was the development of a common European standard for the representation and the exchange of virtual 3D city and regional models. This includes the further development and discussion of the existing ontology and its mapping to an UML data model within the ISO 191xx and OGC frameworks. The mapping to GML3 should ensure the usability within international, national, and regional spatial data infrastructures.