Definition and Manipulation of Graphical Entities in Geographic Information Systems

Geographic information systems have much in common with generalized data base management systems. Both types of systems require the definition of items, attributes, and higher level constructs involving them, e.g. records. Geographic information systems deal in addition with graphical entities, a concept unknown in data base management systems. Not surprisingly, geographic systems have little in common; each one is a new "invention." This paper explores the graphic requirements of two typical geographic information systems with the aim of identifying how such entities are defined, used, and stored in a data base. Examples are drawn from a cartographic system where only graphic display is important, and a query system where graphical entities play an important role in both extraction and display of data. Conclusions are drawn in terms of the shortcomings of generalized data base management systems with regard to data definition, data storage, and data manipulation. The interdependence of these operations is discussed, illustrating that the way in which a graphical entity is manipulated by the user impacts on its definition and storage.