-1 - Three-dimensional (3d) Modelling in a Geographical Database Three-dimensional (3d) Modelling in a Geographical Database

While most current Geographical Information Systems (GIS) are 2D GIS, more and more application domains as geology, civil and military engineering, etc, need 3D GIS. The particularities and capabilities of a 3D GIS are outlined in this paper. Although 3D GIS are not much developped, some studies have already been done, especially in order to represent 3D data for geological or architectural applications. Such 3D models are generally based upon Computer-Aided Design (CAD) models. The use of 3D CAD models for representing 3D in GIS is discussed in the paper. As 3D GIS represent a 3D map, not only the surface of the terrain but also the entities set on the relief, e.g. buildings, have to be represented. The proposed 3D model for a GIS consists of representing 3D entities set on a Digital Terrain Model (DTM). These entities are represented with different approximation levels, the first two being used as index and processes accelerators and the last offering an efficient visualization support. The first two levels are entities decomposition into minimal bounding rectangular parallelepipeds (MBRP). The last level is the external representation: it is a Boundary Representation (BR). Indexation is shortly discussed in the paper. An existing geographical database system is currently extended to support these 3D entities.