This paper introduces a model of spatial cognition to describe the states of partial knowledge that people have about the spatial structure of a large-scale environment. Spatial knowledge has several different representations, each of which captures one aspect of the geography. With knowledge stored in multiple representations, we must examine the procedures for assimilating new information for solving problems, and for communicating information between representations. The model centers on an abstract machine called the TOUR machine, which executes a description of the route to drive the "You Are Here" pointer (a small working memory) through a map that describes the geography. Representations for local and global spatial knowledge are discussed in detail. The model is compared with a survey of the psychological literature. Finally, the directions of necessary and desirable future research are outlined.