Glossary for Geospatial Science

DEM:Digital Elevation Model.  (Also DTM, “Digital Terrain Model”).  A computerized representation of an elevation surface.  Specifically, a raster in which the value in each cell represents the surface elevation at that location in the scene.  DEMs are USGS geographic elevation data distributed in raster form on open reel magnetic tapes.  There are 2 basic types:  1) The DMA type created by the Defense Mapping Agency in both a fixed cell size and a 3 x 3 arcsecond cell size distributed in 1 x 1 degree files.  2) A newer format for those 7.5' USGS quadrangles that have been processed into 1 x 1 arcsecond elevation cells.