DEM's are sampled arrays of elevation values representing ground positions at regularly spaced intervals. Digital Elevation Model (DEM) is the terminology adopted by the USGS to describe terrain elevation data sets in a digital raster form. The standard DEM consists of a regular array of elevations cast on a designated coordinate projection system. The DEM data are stored as a series of profiles in which the spacing of the elevations along and between each profile is in regular whole number intervals. The normal orientation of data is by columns and rows. Each column contains a series of elevations ordered from south to north with the order of the columns from west to east.

The DEM is formatted as one ASCII header record (A-record), followed by a series of profile records (B-records) each of which include a short B-record header followed by a series of ASCII integer elevations per each profile. The last physical record of the DEM is an accuracy record (C-record). 7.5-minute DEM (30- by 30-meter data spacing, cast on Universal Transverse Mercator (UTM) projection). Provides coverage in 7.5- by 7.5-minute blocks. Each product provides the same coverage as a standard USGS 7.5-minute quadrangle without over edge. Coverage is for the Contiguous United States, Hawaii, and Puerto Rico. 7.5-minute DEM's have rows and columns which vary in length and are staggered. The UTM bounding coordinates form a quadrilateral (no two sides are parallel to each other), rather than a rectangle. The user will need to pad out the uneven rows and columns with blanks or flagged data values, if a rectangle is required for the user's application. Some software vendors have incorporated this function into their software for input of standard formatted USGS DEMs.

Figure 1. USGS Digital Elevation Model (DEM)

For the conterminous U.S and Hawaii, primary series DEM's are prepared at two levels: Level 1-derived from manual profiling or image correlation of aerial photographic stereomodels; Level 2-derived from hypsography, either by digitizing USGS primary map contour separates or by stereomodel digitizing. The unit of coverage for DEM's in the conterminous U.S. is the 7.5-minute quadrangle. For Alaska, Level 2 DEM's are prepared from the hypsography overlays of the 1:63,360-scale Alaska primary series map, and the unit of DEM coverage is the Alaska primary series map. All DEM's adhere to the USGS NMPTI Standards for Digital Elevation Models.