Terrain models were constructed for all 141 WDSs. The range between the highest and lowest nodal elevations would be insufficient to describe the terrain. The reason for this is that the range would not account for the number of smaller hills and terrain fluctuations inside each WDS. The range and standard deviation of the node elevations were used instead, in order to describe the terrain. By including the standard deviation, the fluctuations of the nodal elevations were accounted for. Elevation index tables, represented in Tables 3 and 4, were used to categorise each WDS. The range and standard deviation index value were the same for the majority of the WDS models. For zones where this was not the case, an average value was used for terrain classification.