An efficient algorithm for assignment of flow direction over flat surfaces in raster DEMs based on distance transform

Flat surfaces or areas with no local gradient are general types of terrain in raster Digital Elevation Models (DEMs). When a DEM is used for automated hydrological analysis, it is necessary to assign flow directions over flat surfaces. This paper presents an improved algorithm for assigning flow directions over flat surfaces that builds on earlier work and offers efficiency improvements. The improved algorithm use distance transform method to calculate the distance values of cells in flat surfaces towards lower terrain and away from higher terrain, which can replace the recursive process in the earlier algorithm by a linear process, and reduce random data access. Furthermore, the improved algorithm assigns weights with different values and signs to the two distance values to guarantee that all flat surfaces are drained, which avoids the need to label each flat surface using the flood-fill algorithm in the earlier work. Comparisons of speed indicate that the improved algorithm is more efficient in assigning flow directions over flat surfaces.