A rigorous test of the accuracy of USGS digital elevation models in forested areas of Oregon and Washington.
A procedure for performing a rigorous test of elevational accuracy of DEMs using independent ground coordinate data digitized photogrammetrically from aerial photography is presented. The accuracy of a sample set of 23 DEMs covering National Forests in Oregon and Washington was evaluated. Accuracy varied considerably between eastern and western parts of Oregon and Washington, and to a lesser extent, by DEM production method.
The elevational root mean square errors (P, SE) computed from independent ground data for DEMs produced using the line-trace method were on average 4 times larger than RMSEs published by the USGS for both eastside and westside DEMs. Computed RMSEs for all westside line-trace DEMs exceeded 7m; whereas, 80°,/o of eastside line-trace DEMs had computed RMSEs of 7m or less.
Published RMSEs of eastside DEMs produced using the photogrammetric technique agreed within I m on average with RMSEs computed from independent ground data; and, all tested eastside photogrammetric DEMs had RMSEs of 7m or less. Published RMSEs for westside photogrammetric DEMs were on average 4m lower than the computed RMSEs; and, 75% of computed RMSEs exceeded 7m.
Although published RMSEs for line-trace DEMs were on average lower than published RMSEs for photogrammetric DEMs, the RMSEs computed from independent data sets did not follow this pattern: photogrammetric DEMs had lower RMSEs than line-trace DEMs. In addition, published RMSEs of photogrammetric DEMs had a much higher correlation (r2=0.64) than did line-trace DEMs (r2=0.36) with RMSEs computed from independent test data.