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Geographic Information Systems

A Cautionary Note

GISs encourage the user to do things that often are not justified by the nature of the data involved (Goodchild and Gopal 1989). The ability to change map scales and to overlay maps can be deceiving; the user must be aware of the imprecision inherent in all cartography and of the ways errors compound when map scales are changed or when maps are merged (Abler 1987).

The presence of error on maps begins with the process of map projection (Vitek et al. 1984). A map projection is a systematic representation of all or part of the three-dimensional earth to a two-dimensional plane. Since this cannot be done without distortion, the user must choose the map property to be shown accurately at the expense of others, or a compromise of several properties (Snyder 1987). Error is introduced to maps in the process of cartographic abstraction. A map is a model of reality, and map contents are often elegant misrepresentations of changes that are often gradual, vague, or fuzzy (Burrough 1986). Map errors involve both positional and attribute accuracy, which in some instances can be difficult to separate. Furthermore, both position and attribute error are a function of scale.

Errors accumulate during the analysis process in a GIS. Newcomer and Szajgin (1984) demonstrated a method for estimating the error propagation during the map overlay process. They concluded that the accuracy of the final map is a function of the number of map layers, the accuracy of these layers, and the coincidence of errors at the same position from several map layers. The accuracy of spatial databases and the propagation of error during data analysis are complex issues (Newcomer and Szajgin 1984, Vitek et al. 1984, Walsh et al. 1987, Goodchild and Gopal 1989, Lunetta et al. 1991).

The future for the integration of GIS, remote sensing, and expert systems is promising. In 1988, the National Science Foundation announced the formation of the National Center for Geographic Information Analysis, a consortium of universities including the University of California at Santa Barbara, The State University of New York at Buffalo, and the University of Maine at Orono. The Center has outlined a program aimed at the systematic removal of perceived impediments to the adoption and use of GIS technology. The program consists of a series of initiatives, several of which already have resulted in publications and symposia.

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