USGS 30-meter digital elevation models are published with very little general accuracy information, none of which gives the user an estimate of the spatial variability of error in the dataset.  We have done extensive high resolution GPS work at Sedgwick Reserve, and used this "ground truth" data to assess the accuracy of the Los Olivos 30-meter DEM.  The results show there is considerable systematic error in the DEM, which is exaggerated in derivative products, such as the slope or aspect grids.  Depending on the resolution requirements of the user, this can degrade the quality of any analysis done using these grids.

Example of effect of DEM error on grid calculations

The most effective way of assessing how DEM error affects a specific application is using stochastic simulations of the DEM.    Stochastic simulations are alternative grids or realizations generated from the measured distribution of error over the DEM.   These realizations are available for all of the grids distributed here by contacting K. Holmes at the UCSB Department of Geography.

For more information about stochastic simulations, see a basic geostatistics text or try the following links for more information:

Ashton Shortridge, UCSB Department of Geography