The values represent spot elevations on a rectangular grid in the UTM projection with 30-meter horizontal spacing in both east-west and north-south directions . The USGS documentation describes the generation and structure of DEMs in detail. When inported into ArcInfo (using, for example, DEMLATTICE) or LAS (DEM2LAS), the grid points are placed at the center of each 30m by 30m grid cell or pixel.
Note that the rows and columns of the 30-meter elevation grid are parallel to lines of constant northing and easting in the UTM projection. They are not, in general, parallel to the lines of constant latitude and longitude which define the edges of the 7.5-minute quadrangle. As a result, when the maximum and minimum UTM easting and northing values for the elevation grid points are used to define a rectangle in the UTM projection, this rectangle contains narrow wedges along its edges which are outside the quadrangle boundaries. Elevation values are not given for grid points falling within these wedges. When data for adjacent quadrangles are seamed together, however, using standard image processing or GIS software such as LAS or ArcInfo, these wedge-shaped areas will be correctly filled from the adjacent quadrangles so long as they are in the same UTM zone.
The file name extension (.dem or .sdts) indicates whether the file is in traditional USGS ASCII format or the new Spatial Data Transfer Standard (SDTS) format, respectively. For SDTS files only, the processing level is appended to the name of the map quad, since in some cases the USGS distributes more than one version of the DEM. In most cases, a higher level number implies greater overall quality of the data. There may also be different versions of the same DEM which use different units for the elevation values. SDTS file names for which the elevation is in feet have "_ft" appended to them. For elevations in meters, which is the default, the file name does not include a units flag.
The elevation grid is in the UTM map projection, zone 18.