The area of coverage of any particular NED layer is generally determined by the availability of source elevation data of similar or higher resolution.
A digital elevation model, or DEM, is a representation of the terrain (bare-earth) with elevations on the ground at regularly spaced intervals. A digital surface model, or DSM, also contains elevations at regularly spaced intervals.
A digital terrain model, or DTM, may be a different type of model from a DEM, such as an irregularly spaced vector model of bare earth points.
The laser pulses emitted from terrestrial lidar systems can be reflected from man-made structures, vegetation, or the earthen surface.
The National Elevation Dataset (NED) serves the elevation layer of The National Map, and provides basic elevation information for earth science studies and mapping applications in the United States.
The most recently published figure of overall absolute vertical accuracy of the NED within the conterminous United States, expressed as the root mean square error (RMSE) of 25,310 reference points, is 1.55 meters.
There are two types of metadata available for the NED:  (1) spatial metadata, and (2) FGDC metadata. Spatial metadata in the form of a shapefile is supplied as a part of each NED data download, or may be obtained at the
NED data can be ingested by most commercial GIS software.
Shapefiles delineating update areas can be found at the NED Metadata download page.