USGS ASCII DEM files.
Standard format for the distribution of terrain elevation data for the United States.
United States Geological Survey (USGS) standard.
DEM is an acronym for Digital Elevation Model.
Typically distributed as .zip or .gz compressed file.
The .dem file extension is often omitted.
Common variants are 1-degree and 7.5-minute-degree DEMs.
1-degree DEMs cover 1×1-degree blocks at a 3-arc-second grid spacing and 1201×1201 raster size.
The 1-degree USGS DEM format is occasionally referred to as 1:250,000 scale DEM.
7.5-minute DEMs cover 7.5×7.5-minute blocks at a 30-meter grid spacing and correspond to the USGS 1:24,000 and 1:25,000 scale topographic quadrangle map series.
The raster coordinates used for 7.5-minute DEM files are referenced in the Universal Transverse Mercator (UTM) projection, resulting in irregular raster dimensions.
Elevations are given in meters relative to the National Geodetic Vertical Datum of 1929 (NGVD) for the continental United States and relative to the local mean sea level in Hawaii.