Exploring the effects of curvature and refraction in GIS-based visibility studies

The curvature of the earth affects visibility studies when using a DEM (Digital Elevation Model) and needs to be taken into consideration as pointed out by Conolly and Lake in 2006, although little work has been done in archaeology to explore the implications of this warning. This paper addresses related methodological issues in visibility studies namely the issue of reality, of edge effects, and of significance. We use the long barrows of the Danebury region in England as a case study. New methods to calculate and assess the combined effects of curvature and atmospheric refraction on visibility studies are presented. It is shown that within a visibility range of 10km there is a decrease in visibility of approximately 20% for the Danebury landscape; the effect is larger at areas closer to the horizon.