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 first converted all elevation DEM values that were between zero and one to the whole integer value of one. Then we converted all values to the nearest whole integer, and last we 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 arc second DEM creating a 15 arc second version of the DEM that was used in our modeling process.