Viewshed and visibility index are two important topographic data and widely used in earth science, civil engineering and military applications. In this paper, the terrain dominance over the distribution of viewshed and visibility index is studied using DEM of two sets areas and a fast algorithm for viewshed calculation. The distribution of viewshed and visibility index, as might be expected, are found to be strongly influenced by both relative and absolute position. In rugged areas, the viewpoints at convex positions such as ridges and peaks have more visible area and widely spread viewshed than other locations. However, in plateau-like areas, these locations may have less visible area than others.
Using a fast algorithm, the visibility index for every point on both DEMs are calculated. The visibility index of viewpoints, which are 2m and 5m over the ground surface, are also calculated. The distribution of viewshed and visibility index of locations in plateau-like areas are found to be more sensitive to the height of the viewpoint than in rugged areas.