SUMMARYThe needs of three-dimensional implementation and WWW-based geographic information systems are increasing. In this paper, we propose a flexible and efficient VRML-based terrain model. In the proposed model, by placing side by side the terrain models (tiles) of fixed size in 3D space, the simplification of display range change, the utilization of a caring mechanism, and the attachment of an automatic changeover mechanism for the level of detail of the model will become possible. Each tile is modeled by combining multiple grid models, and the useless polygons in the sea and lakes are curtailed. Moreover, we propose an automatic changeover mechanism of the level of detail of the model, which regenerates the shape features of the terrain objects such as square and flat. Furthermore, for curtailing the transmitted data, we also propose a method for automatically generating simplified models from the detailed model on WWW terminal side. Finally, the effectiveness of the proposed model is shown by 3D spatial navigation experiments using automatic viewpoint motion.