The Gap Analysis Program (GAP) is a nationwide effort under the direction of the Biological Resources Division of the U.S. Geological Survey of the U.S. Department of the Interior. It is the first time in the history of the United States that a comprehensive effort has been made to inventory and computerize the kinds and geographic distributions of species of plants and animals that contribute to our national diversity. GAP is being done on a state-by-state basis and currently is underway in all 50 states. Using the methods of GIS, maps of distributions of butterflies, amphibians, reptiles, birds, mammals and threatened, endangered, or sensitive species and their habitats, typically are overlaid upon maps of public lands. Those geographic areas where there are high varieties of species not occurring on such lands are considered "gaps" in a total system of land ownership that can be assumed to be in place to provide for long-term conservation of biological diversity, among other purposes.