Digital elevation models meet horizontal National Map Accuracy Standards (NMAS) accuracy requirements. Additional information on DEM specifications can be found in the USGS National Mapping Program's Standards for Digital Elevation Model (DEMs): <http://mapping.usgs.gov/standards>, and in the USGS publication titled 'Digital Elevation Models - Data Users Guide 5.': <ftp://mapping.usgs.gov/pub/ti/DEM/demguide>.
As stated in the USGS DEM Data User's Guide, 'The method of determining 7.5-minute DEM accuracy involves computation of the root-mean-square error (RMSE) for linearly interpolated elevations in the DEM and corresponding "true" elevations from the published maps. Test points are well distributed, are representative of the terrain, and have "true" elevations well within the DEM accuracy criteria.'
DEMGW930 is a 'composite' 30-meter DEM including Level 2 coverage where it exists and Level 1 elsewhere.
According to the USGS DEM Data User's Guide, "Level 1 DEM's are elevation data sets in a standardized format. The intent is to reserve this level for 7.5-minute DEM's or equivalent that are derived from scanning National High-Altitude Photography Program, National Aerial Photography Program, or equivalent photography. A vertical RMSE of 7 m is the desired accuracy standard. A RMSE of 15 m is the maximum permitted."
"Level 2 DEMs are elevation data sets that have been processed or smoothed for consistency and edited to remove identifiable systematic errors. DEM data derived from hypsographic and hydrographic data digitizing, either photogrammetrically or from existing maps, are entered into the level 2 category after review on a DEM Editing System. An RMSE of one-half contour interval is the maximum permitted. There are no errors greater than one contour interval in magnitude. The DEM record C contains the accuracy statistics acquired during quality control." For more information on the filtering process for 7.5-minute DEMs see: <http://edcnts12.cr.usgs.gov/ned/filter/index.html>
These figures do not include any additional error that may have been introduced in the course of data format conversions and re-projection.
Additional information on DEM specifications can be found in the USGS National Mapping Program's Standards for Digital Elevation Model (DEMs): <http://www-nmd.usgs.gov/www/html/2nmpgds.html>, and in the USGS publication titled 'Digital Elevation Models - Data Users Guide 5': <ftp://mapping.usgs.gov/pub/ti/DEM/demguide>.
Additional information on DEM specifications can be found in the USGS National Mapping Program's Standards for Digital Elevation Model (DEMs): <http://mapping.usgs.gov/standards>, and in them USGS publication titled 'Digital Elevation Models - Data Users Guide 5.': <ftp://mapping.usgs.gov/pub/ti/DEM/demguide>.