Population estimations and indices based on roadside or spotlight counts typically require investigators to determine the route with maximum viewing area and calculate area visible from the census route. Viewshed analysis may provide a simpler and more accurate method for determining route, area and population estimate. From September 2000 to August 2001 we observed adult bison from a driving route at The Nature Conservancy's Tallgrass Prairie Preserve. We recorded group locations to field test the ability of viewshed analysis to predict the area where bison were visible. We developed viewsheds based on digitized layers of the driving route and for high, low and random points within the Preserve to determine the viewshed with maximum visible area. We developed a population estimate based on number of bison observed and visible area calculated from our driving route. We compared population estimates to known bison populations before and after 1 May 2001 and for our entire study period. Ninety-nine percent (P < 0.001) of bison locations occurred within the viewshed. Our driving route provided the most visible area (81%), followed by viewsheds of high (77%), random (61%) and low (31%) points. Estimated adult bison populations before 1 May 2001 were different (P < 0.001), but similar on and after 1 May 2001 (P = 0.10) to known populations. The estimated population for the entire study was different (P < 0.001) from the mean population size. Viewshed analysis is an efficient method to predict locations of easily observed ungulates and has utility in planning census routes with maximum visible area a priori. We suggest that viewshed analysis provides a more accurate estimation of area visible from census routes and resulting population estimations than conventional surveys.
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Vol. 152 • No. 2