Test points are well distributed, representative of the terrain, and have true elevations with accuracies well within the DEM accuracy criteria. A minimum of 28 test points per DEM is required to compute the RMSE, which is composed of a single test using 20 interior points and 8 edge points. These errors are reduced in magnitude by editing but cannot be completely eliminated. Blunder errors are those errors of major proportions and are easily identified and removed during interactive editing. Systematic errors are those errors that follow some fixed pattern and are introduced by data collection procedures and systems.
Random errors result from unknown or accidental causes.
DEM's are edited to correctly depict elevation surfaces that correspond to water bodies of specified size. A RMSE of 7-meters or less is the desired accuracy standard. A RMSE of 15-meters is the maximum permitted. Systematic errors that are within stated accuracy standards are tolerated. This process step describes, in general, the process used in the production of standard USGS DEM datasets.