A comparable measure of view was needed to better evaluate the locational properties of sites encountered on survey. Based on research for a Master's paper on Cumulative Viewsheds in the Ilave Valley (Tripcevich 2001), the work of Wheatley (1995) and Lake et al. (1998;Lake and Woodman 2003), and using methods suggested by Nathan Craig (2000 pers. comm.), a surface was calculated that quantifies the visibility and, to some extent, environmental exposure of a given location.

Figure5-10. Line of sight across hilly terrain results in specific cells and targets being in view or out of view.

The basic concept to cumulative viewshed analysis is that a large number of viewsheds for random locations are calculated and overlaid on one another, and the locations that have a high incidence of visibility from random locations are likely to have both broad viewsheds and an unusually high environment exposure. Comparability between viewshed and exposure has been addressed more thoroughly by Kvamme (1988: 335-336;1992: 26-27) where the exposure is considered in terms of the volume of a cylinder surrounding the point of interest, and thus exposure of a location to a greater variety of altitudes and directions is considered. This approach is more thorough than a visibility calculation, but given the relatively coarse resolution of the DEM, the index for Visibility was assumed to be essentially representative of climatic exposure.

Figure5-11. Viewshed is not necessarily reciprocal, as the individual and the left can see the person on the right, but the opposite is not true.

The assumption is that of reciprocal visibility: if person A can see person B, then person B can see person A. The resulting surface contains a value in each grid cell that indicates how many of the random observers can see that grid cell.