The accuracy of the National Elevation Dataset (NED) varies spatially because of the variable quality of the source digital elevation models (DEMs). As such, the NED inherits the accuracy of the source DEMs. Some accuracy statistics are available in the header records of the USGS DEM source files, and this information is captured in the spatially referenced metadata. Many times, this accuracy information has limited usefulness because it is a relative measure of how well the DEM fits the source material from which it was generated. In an effort to provide more information to users on the vertical accuracy of the NED, the data set has been tested by comparing it with an independent reference source of very high accuracy. The reference data are the geodetic control points that the National Geodetic Survey (NGS) uses for gravity and geoid modeling (Smith and Roman, 2001; National Geodetic Survey, 2003). The distribution of this set of more than 13,000 high-precision survey points across the conterminous United States is shown in Figure 1.

NED Accuracy Document

Figure 1. Reference control point data set used for accuracy assessment of the NED.