Using the assumptions about the error distribution cited above, a RMSE of 97 meters can be expressed as 160 meters linear error at 90 percent confidence. The estimate was derived from the dominant implied contour interval in the grid of 20m, plus the dense coverage of source data compared to the 30 o output grid. It is unknown if any of these maps actually meet this standard. Also, map digitizing and elevation surface interpolation errors are unknown and therefore not included. However, histogram analysis of DEMs derived from cartographic sources of undocumented contour interval can often be used to estimate contour intervals from those original cartographic sources.
Histogram analyses of DEMs can often determine the likely contour values of source maps.
These interpreted contour intervals were halved to estimate vertical accuracy of DEMs. In many cases, more than one contour interval appears to exist in DEMs, perhaps because the DEMs were produced from various types of maps, or perhaps because of supplemental contour intervals. In such cases, vertical accuracy was estimated conservatively. Nevertheless, the estimates noted above are probably reasonably accurate.