Visual impact studies often make use of Boolean viewshed maps created by standard functions implemented in most raster GIS software. The DEM is used in a deterministic way and possible inaccuracies in the DEM are neglected. Monte Carlo simulations of the errors in the digital elevation data (DEM error) can be used to test the effect of the DEM error on the calculated viewshed. Both an explorational and a quantitative method based on binomial statistics are proposed to analyse the impact of the number of simulations on the accuracy of the probabilistic visibility of a cell. The probabilistic information is used for the creation of a Boolean viewshed map with a known user's and/or producer's accuracy. The importance of this methodology is illustrated.