Simulation of the Uncertainty of a Viewshed

One of the most widely available procedures packaged with CIS for the analysis of a Digital Elevation Model (DEM) is the identification of the viewable area, or the viewshed. The elevations recorded in the DEM do, however, contain error, and the USGS, for example, publishes a Root Mean Squared Error (RMSE) for each DEM. Research reported here assesses the uncertainty of locations being within a viewshed, given the published error for the DEM. In this research, repeated error fields are simulated with variable spatial autocorrelation, and added to the original DEM. The viewshed is then determined in the resulting noisy DEM. Results show that using the basic assumption of spatial independence in the error which is implicit in the RMSE remarkably few points are reliably within the viewshed. With spatially autocorrelated noise, the reliability is higher, but still should be cause for concern to many using viewshed procedures.