An Efficient Algorithm for Depression Filling and Flat-Surface Processing in Raster DEMs

Depressions (or pits) and flat surfaces (or flats) are general types of terrain in raster digital elevation models (DEMs). Depressions are lower areas surrounded by terrain without outlets, and flat surfaces are areas with no local gradient. To extract hydrologic or geomorphic properties from DEMs, these two types of terrain need to be addressed. This letter presents an efficient algorithm for filling depressions and for adding increments to flat surfaces. The algorithm builds on previous work, offering several important improvements. The improved algorithm uses two queues: a priority queue and a “first-in, first-out” (FIFO) queue. The FIFO queue is used to process depressions and flat surfaces, and the priority queue processes other terrain. The improved algorithm achieves an O(M log₂ M) time complexity, where M is less than the total number of cells, which is more efficient than the algorithm proposed by Wang and Liu. In addition, the improved algorithm not only fills depressions but also elevates flat surfaces for the convenience of extracting flow directions. Furthermore, to adapt to different data types, for example, integer, single-precision floating point, and double precision, the improved algorithm does not alter flat-surface elevations in DEMs directly but uses a mask matrix to mark the incremental elevation values of flat surfaces. In speed comparison testing, the improved algorithm performed up to 16%-32% faster than the original.