The QC (quality control) checkpoints should be selected on flat terrain, or on uniformly sloping terrain for x-meters in all directions from each checkpoint, where "x" is the nominal spacing of the DEM or mass points evaluated. Whereas flat terrain is preferable, this is not always possible. Whenever possible, terrain slope should not be steeper than a 20 percent grade because horizontal errors will unduly influence the vertical RMSE calculations. For example, an allowable 1-meter horizontal error in a DEM could cause