Elevation

National Elevation Dataset (NED) Release Notes were produced through February 2014 to provide details about bimonthly releases of the National Elevation Dataset (now referred to as 3DEP seamless DEMs).
All 3DEP seamless DEMs are provided in geographic coordinates (longitude and latitude) in units of decimal degrees, horizontally referenced on the North American Datum of 1983 (NAD83).
Seamless DEMs (2-, 1-, and 1/3-arc-second) are available in ArcGrid, GridFloat and IMG formats. Both of the 1/9 arc-second and 1 meter data are available in IMG format. Original product resolution DEMs are available in ArcGrid format.
The NED data is still available and being updated continually, but the ‘NED’ terminology has been retired. The seamless national DEM layers previously referred to as the ‘NED’ are now branded as 3DEP Seamless DEMs.
The area of coverage of any particular standard 3DEP DEM product 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 at regularly spaced intervals.
The 5-meter DEMs in Alaska contain ‘DTM’ in the file names.
The laser pulses emitted from airborne lidar systems may be reflected from man-made structures, vegetation, or the earthen surface.
The most recently published figure of overall absolute vertical accuracy of the seamless DEMs within the conterminous United States, expressed as the root mean square error (RMSE) of 25,310 reference points, is 1.55 meters.