Siting Observers on Terrain

This paper presents an experimental study of a new algorithm that synthesises separate programs, for fast viewshed, and for fast approximate visibility index determination. It has been implemented in a working testbed for siting multiple observers jointly to cover terrain from a full level-1 DEM, and to do it so quickly that multiple experiments are easily possible. Both the observer and target may be at a given fixed distance above the terrain. The process operates as follows. (1) An approximate visibility index is calculated for each point in the cell under consideration. (2) A set of tentative observers is selected from the highly visible points. (3) The actual observers are selected from them, so as to cover as much of the cell as possible, using a greedy algorithm. Various experiments with varying parameters were performed on the Lake Champlain West cell (raster data set), with observations such as the following. (1) Forcing tentative observers to be well spaced was more important than using the most visible tentative observers. (2) Most of the new observers added (because they covered the most unseen points) were already visible to an existing observer. (3) Randomly deleting many tentative observers before final selection did not reduce the final area covered.