Topology revisited: representing spatial relations

Topology is a central, deŽ ning feature of geographical information systems (GIS). The advantagesof topological data structures are that data storage for polygons is reduced because boundaries between adjacent polygons are not stored twice, explicit adjacency relations are maintained, and data entry and map production is improved by providing a rigorous, automated method to handle artifacts of digitizing. However, what explains the resurgence of non-topological data structures and why do contemporary desktop GIS packages support them? The historical developmentof geographicaldata structures is examined to provide a context for identifying the advantages and disadvantages of topological and non-topological data structures. Although explicit storage of adjacent features increases performance of adjacency analyses, it is not required to conduct these operations. Non-topological data structures can represent features that conform to planar graph theory (i.e. non-overlapping, space-Ž lling polygons). A data structure that can represent proximal and directional spatial relations, in addition to topological relationships is described. This extension allows a broader set of functional relationships and connections between geographical features to be explicitly represented.