Residual relief separation: DEM enhancement for geomorphological mapping

Geomorphologically mapped data form a primary set of observations that can be used to infer former environmental conditions. Thus, objective and consistent mapping of landforms from remotely sensed data (e.g. satellite imagery, digital elevation models (DEMs) is paramount for reconstructing palaeo-environments. This paper proposes a technique, “residual relief separation”, to enhance landforms in DEMs prior to visualization and digital mapping. This is applied to a ~600 km 2 region surrounding Lough Gara, Ireland, where drumlins (~200 m wide) overlie a regional relief of hills (~10 km wide). Here, residual relief separation uses this difference in width-scale. Regional relief is approximated by a 1 km wide median filter, then subtracted to leave the drumlins in a “residual” topography. In a second step, the residual relief is normalized to allow for amplitude variations in the drumlins across the area (~5-40 m high). Finally, visualization uses a simple black-to-white colour scale for height. Whilst not numerically out-performing other visualization techniques, this method performs equally well, and as the data are not “illuminated” there is no azimuthal bias. Additional benefits include the relatively simple calculation, intuitive visual comprehension, no emphasis of noise, and the possibility of using any desired visualization technique after the landscape has been topographically manipulated.