Abstract: Terrain viewshed analysis is an important content of spatial analysis．Based on DEM,the current algorithms have complexity in computation and bad applicability.Especially according to the multiresolution DEM data,they are immature this paper,from organization of terrain data and the necessity of defilade angles analysis,considering the influence of the data structure on the algorithm, discussed the main contents of the calculation principles of defilade angles and key technologies of terrain viewshed analysis in detail．DEM data was organized and managed with Quad-tree structure based on the Pyramid model in this paper．The matrix called “Advanced Matrix” was introduced so that the calculation efficiency was highly improved．Based on those above,the advanced algorithm based on DEM was used to do the analysis of terrain viewshed angles automatically．

Key words: Viewshed,Defilade angle,DEM,Quad-tree model,Advanced matrix