Many difficult decision-making processes in geographic planning, environmental, and social studies arise because they require the manipulation and integration of large volumes of spatially-indexed data. Computerized geographical information systems (GIS) provide the key to the efficient use of such data sets because they can efficiently perform such functions on vast quantities of spatially addressed data. These powerful systems acquire data from many sources; change the data into a variety of useful formats; store the data; retrieve and manipulate the data for analysis; and then generate outputs required by a given user.

Underlying each GIS is a powerful spatial database management subsystem that provides such functions as efficient data storage, retrieval and updating, as well as security, integrity, and redundancy control. Within the database, a variety of mappable characteristics associated with any given geographical location are organized as a series of spatially-registered maps, or "overlays...