Mapping Viewsheds—Time

U.S. Department of Agriculture, Forest Service

These maps describe a process used to streamline viewshed mapping of more than 13,000,000 acres within the national forests of Montana and Idaho.

Testing of the Viewshed tool in Spatial Analyst determined that the time to complete the analysis is directly proportional to the number of viewpoints and the number of cells being mapped. With the given project, conventional analysis would take about 20,000 hours or two and one half years to compute but the project was only funded for about fifty days.

A solution was to use surface roughness to approximate viewed zones and then model viewed areas at a much coarser scale. The two models were then combined, using the coarse visibility to mask viewed zones at the middle ground and background levels. Using this method is twenty times faster than the conventional viewshed computation.

The model (Model results—3 hours map) creates variable boundaries between foreground, middle ground, and background which better represents the on-the-ground experience. It also maps more areas as seldom seen by dropping out areas that are only seen by a few, of the many viewpoints.

The results map ("True" viewsheds—58 hours) shows the results of a complete viewshed mapping exercise, using each of the three levels of route concern. This map took 58 hours to compile and represents only a very small portion of the forest.

Courtesy of U.S. Department of Agriculture, Forest Service.

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