Digital elevation model (DEM) data consist of a sampled array of regularly spaced elevation values referenced horizontally either to a Universal Transverse Mercator (UTM) projection or to a geographic coordinate system. The grid cells are spaced at regular intervals along south to north profiles that are ordered from west to east. The U.S. Geological Survey (USGS) produces five primary types of elevation data: 7.5-minute DEM, 30-minute DEM, 1-degree DEM, 7.5-minute Alaska DEM, and 15-minute Alaska DEM.

The USGS has used four methods to collect DEM data. Of these methods, only one, interpolation from vectors or digital line graph (DLG) hypsographic and hydrographic data, is currently used for7.5-minute DEM's and other series DEM's. The following three methods (now discontinued or deactivated) were used primarily for the production of 7.5-minute DEM data: (1) the Gestalt Photo Mapper II;(2) manual profiling from photogrammetric stereomodels; (3) interpolation of the elevations from stereomodel digitized contours.
DEM data for 15-minute and 2-arc-second (30 minute) units are derived from DLG hypsographic and hydrographic data. DEM data for 1-degree units are collected from topographic map sources, ranging from the 7.5-minute map series to the 1by 2-degree map series, or from photographic sources by using image correlation systems.
Unit Size and File Extent
DEM data for 7.5-minute units correspond to the USGS 7.5-minute topographic quadrangle map series for all of the United States.
Data for 2-arc-second (30 minute) DEM's provide coverage for the conterminous United States, except Alaska. This series is distributed as four units of 15-minute DEM data covering full 30by 30-minuteareas that correspond to the east or west half of the USGS 30by 60-minute topographic quadrangle map series (1:100,000 scale).
DEM data for 15-minute units correspond to the USGS 15-minute topographic quadrangle map series in Alaska. The unit sizes in Alaska vary depending on the latitude. Units south of 59ù N. cover 15by 20-minute areas, those between 59ù and 62ù N. cover 15by 22.5-minute areas, those between 62ùand 68ù N. cover 15by 30-minute areas, and those north of 68ù N. cover 15by 36-minute areas. (All values are latitude-longitude, respectively.)
DEM data are produced by the National Imagery and Mapping Agency (NIMA) in lby 1-degree units that correspond to the east or west half of USGS 1by 2-degree topographic quadrangle map series(1:250,000 scale) for all of the United States and its territories. In Alaska these are west, central, and east files.
All nonstandard quadrangles with neat-lines that extend beyond the standard unit size to accommodate overedge boundaries are collected and sold as multiples of the standard unit sizes.