An automated approach to determine optimal parameters for geomorphons mapping
Geomorphons is a cell-based DEM classification method for the identification of landform elements at a broad range of scales by using line-of-sight based neighbourhoods. However, to achieve the advantages of its robustness and efficiency several essential parameters need to be set optimally for the given area of interest. The characteristic of geomorphons map depends on the value of lookup distance defining the maximum scale of mapping, the skip radius to eliminate forms that are too small to be of interest and the flatness threshold to prevent the analysis of flat areas.
We introduce a new automated approach – implemented as a bash shell script for GRASS GIS – to determine these specific thresholds. This approach relies on the potential of the topographic grain to detect the characteristic local ridgeline-to-channel spacing. Calculating the relative relief value with increasing neighbourhood matrices it is possible to define a break-point where the growing of local relief encountered by the sample increases much less. The flatness threshold was defined based on the flatness (ln(S1/S2)) value calculated according to the eigenvector method. Flatness is especially important with low resolution DEMs as even a smaller slope difference for a longer side cell means several meters of height difference.
The proposed method has been applied to freely available SRTM DSMs in two different terms of roughness: dune fields and hilly terrain. Tests provided satisfactory results for generic applicability. This approach has a significant potential for enabling objectivity and automation of landform mapping.