Calculating Ridgeways – Today and in the Past

Irmela HERZOG

(Germany)

The book “Beyond the Map” edited by Gary Lock in 2000 contains two papers on least-cost paths which have had a considerable impact on GIS applications in archaeology. The first paper by Marcos Llobera presents a slope-dependent cost curve for walkers which has been cited and used by many other studies. In addition, the paper models not only natural costs but also social factors that might influence least-cost paths. The second paper by Tyler Bell and Gary Lock compares GIS generated least-cost paths and the prehistoric route of the Ridgeway in Oxfordshire. A complicated topographical bias function was necessary to keep the path on the ridge. This example is cited by handbooks and other studies to show the problems involved in least-cost path calculations. In addition, this paper introduced the concept of effective slope, i.e. calculating costs on the basis of a slope map is not good enough, some correction is needed if the traveller traverses a hillside at angles near-perpendicular to the slope.
A new least-cost path study of ridgeways will be presented in this paper focusing on the region Bergisches Land in North Rhine-Westphalia (Germany). The historic routes in this area are supposed to follow the ridges and to avoid larger creeks and wet valleys. New slope-dependent cost functions will be presented for pedestrians and for vehicles. It will be shown that selecting appropriate cost functions can obviate the need for complicated corrections using a topographical bias function. But even today the cost functions available are far from perfect because it is difficult to include all factors of relevance in route selection and to assign an appropriate weight to each factor.

Keywords: Least cost paths, DEM, ridgeway