ABSTRACT
Geographic information systems permit a wide range of operations to be applied to spatial data in the production of both tabular and graphic output products. Too frequently, however, these operations are applied with little regard for the types and levels of error that may result. In numerous published articles detailing GIS applications, a critical examination of error sources is conspicuously absent and output products are presented without an associated estimate of their reliability. Unfortunately, in most cases these omissions do not imply that errors are of a sufficiently low magnitude that they may safely be ignored. Moreover, the fact that input data are themselves of relatively high quality is no guarantee that output products will be error-free.
