Towards Unified 3d City Models

An increasing number of municipalities decide nowadays to build up 3D city models. Often the main purpose of such a model is to support urban planning processes. However, in most cases this support currently is restricted to the visualization of virtual scenes. The first reason is that there are still no commercial 3D geoinformation systems available. Thus, city models typically are implemented on top of CAD systems or visualization software which all offer only limited modeling capabilities. The second reason is that there does not exist a standard for 3D city models yet. Only few investigations about multifunctional and multiscale modeling, storage and analysis have been carried out so far. In this paper we propose a unified model for the representation of spatial objects in 3D city and regional models. It constitutes a base schema providing patterns for application specific 3D models. It is shown how real world objects are represented by features with geometric, topological and thematic (i.e. non-spatial) properties. We explicitly cope with the problem of multiscale representations. A special level-detail-of-relation between features and their geometry is introduced ensuring spatial consistency between 3D models at different scales. Furthermore, issues concerning the integration of features below surface with the digital terrain model are discussed. Finally, we show how interoperability at system level can be achieved by mapping the proposed model to GML3, the new standard for the representation and exchange of spatial data developed by the OpenGIS Consortium.