Abstract:Visibility analysis is widely applied to military and project fields, of which viewshed computing is the important factor. This article discusses the common algorithms for viewshed computing, analyses the advantages and disadvantages of them, and puts forward a improved Xdraw algorithm based on the old one to reduce obviously the frequency of access to database aiming at a large scale DEM database. Then the continuous research direction is shown through analysis of this algorithm from correctness and efficiency in practice.