Spatial data are often stored in a database with the finest level of details. The problem of cartographic generalization of spatial data concerns deriving spatial data with proper level of details when the data are used for an application. The generalization process is not simply a process of selecting objects or parts of objects suitable for an application. It should maintain the semantic information carried with the spatial data, such as topological relationships among objects and the overall "look and feel" of the data after generalization. Most difficulties of map generalization come from the fact that the importance of objects, which determines whether an object should be included and how it should be visualized in the generalized data set, is decided not only by their sizes, but also the "meaning" of these objects to the application. The latter is application