With the recent increase of demands for 3D information, we need a robust data model of 3D spatial objects to meet the requirements from diverse applications. For this reason, several studies have been done out by ISO/TC211 and OGC, among which the data model of ISO 19107 and the data model for KML are the most important ones. The data model of ISO 19107 provides a strong expressive power of 3D spatial information with a sophisticated model for full 3D solid object. And GML is based on the spatial data model of ISO 19107. However this data model has a serious drawback that the size of data in GML is large and the implementation of ISO 19107 is difficult and heavy, due to the complicated structure of this data model.

On the contrary, the data model employed by KML, which has been developed for the use of Google Earth, includes a simple 3D spatial data model to offer visualization services of 2D and 2.5D spatial objects. Compared with GML, the size of data in KML is smaller than GML and most of systems supporting KML are lighter than those for GML.  However the expressive power in KML is limited due to its simple spatial data model and COLLADA, which is another data format for 3D spatial objects, must to be used if the 3D information is complex. 

In this paper, we propose an alternative 3D data model, called prism model, to provide an enough expressive power and achieve a satisfactory efficiency at the same time. Note that the prism means not only the triangular prism but also polygonal prism. Our model is based on the extrusion technique to represent 3D objects from 2D footprint spatial objects like the model in KML. But we generalized it to handle more diverse shapes with upper and lower geometries as shown in figure 1.