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A geographic information system, or GIS, is a computerized data management system used to capxure, store, manage, retrieve, analyze, and display spatial information. Data capxured and used in a GIS commonly are represented on paper or other hard-copy maps. A GIS differs from other graphics systems in several respects. First, data are georeferenced to the coordinates of a particular projection system. This allows precise placement of features on the earth’s surface and maintains the spatial relationships between mapped features. As a result, commonly referenced data can be overlaid to determine relationships between data elements. For example, soils and wetlands for an area can be overlaid and compared to determine the correspondence between hydric soils and wetlands. Similarly, land use data for multiple time periods can be overlaid to determine the nature of changes that may have occurred since the original mapping. This overlay function is the basis of change detection studies across landscapes.