The U.S. Geological Survey has developed a National Elevation Dataset (NED). The NED is a seamless mosaic of best-available elevation data. The 7.5-minute elevation data for the conterminous United States are the primary initial source data. In addition to the availability of complete 7.5-minute data, efficient processing methods were developed to filter production artifacts in the existing data, convert to the NAD83 datum, edge-match, and fill slivers of missing data at quadrangle seams. One of the effects of the NED processing steps is a much-improved base of elevation data for calculating slope and hydrologic derivatives. The specifications for the NED 1 arc second and 1/3 arc second data are: Geographic coordinate system Horizontal datum of NAD83, except for AK which is NAD27 Vertical datum of NAVD88, except for AK which is NAVD29 Z units of meters
The PRISM Group at Oregon State University converted all values to the nearest whole integer, and then applied a Gaussian filter (Barnes, S.L. 1964: A technique for maximizing details in numerical weather map analysis. J. Appl. Meteorol., 3, 396-409.) to the original NED 1/3 arc second DEM creating a 3 arc second version of the DEM that was used in our modeling process.