GIS is used to aid in the performance of a number of tasks, such as calls for police assistance, emergency response planning, determination of police beats, the study of changes in crime patterns over time, and a host of other applications. A major component is the ability to produce maps. Mapping data enables crime analysts, beat patrol officers, and the general public to interpret information in new ways. A GIS may be defined as a process in which geographic, or locational data are acquired and compiled into a digital database consisting of both base maps and the characteristics, or attributes, of the base map features. A GIS database, comprising maps and attributes of the maps features, is usually expressed in one of two data models: vector or raster. The vector data model shows features on the earth represented as points, lines, or polygons. The raster data model subdivides the study area into a grid made up of square cells, or pixels. Each cell, covering a small geographic area, contains a single attribute value. The vector model displays maps in a format that is familiar to the general public, but vector-based GISs can be imprecise when analyzing certain types of data. Raster-based GISs are useful for mapping data that vary continuously over a surface, such as crime densities, but tend to be large files, and output displays are difficult to interpret unless there is some vector overlay. Using either data model, an analyst can pose questions of the geographic database. One of the hallmarks of a GIS is the ability to overlay different map layers, or themes, to study the relationships among the various layers. A nationwide crime mapping survey began in March 1997 to determine which law enforcement agencies use GIS technology. According to the survey, automated pin mapping is the most frequently used analysis technique. Most of the departments that conduct hot spot analyses of crime locations visually identify hot spots. 13 figures, 3 endnotes, 19 references