Parallel assignment of flow directions over flat surfaces in massive digital elevation models

Abstract

The assignment of flow directions in flat regions needs to be treated with special algorithms to obtain hydrologically correct flow patterns. Based on the sequential algorithm and the three-step parallel framework, we propose a parallel algorithm for assigning the hydrologically correct flow directions in flat regions. The proposed parallel algorithm assigns pre-divided tiles to multiple consumer processes, which construct local graphs that encode the geodesic distance information among tile border flat cells and higher terrain or lower terrain. Based on the local graphs in all tiles, the producer process constructs the global graph and computes the global geodesic distances of tile border flat cells from higher terrain and lower terrain. The consumer processes then compute the final geodesic distances in each tile and assign the hydrologically correct flow directions. Four experiments are conducted to evaluate the performance of our proposed algorithm. The speed-up ratios are greater than 3 when the number of consumer processes is greater than 9. When the number of consumer processes reaches 20, the average strong scaling efficiency is around 40%. The weak scaling efficiency is lower than 20% when the number of consumer processes is greater than 5. Compared with the sequential algorithm that is simpler to implement, the proposed parallel algorithm requires less memory on consumer processes than the sequential algorithm and can process massive digital elevation models that cannot be successfully processed using the sequential algorithm. It fills the void in the processing pipeline of automatic drainage network extraction based on the three-step parallel framework and enables parallel implementation of the entire processing pipeline possible, which should substantially improve the attractiveness of the three-step parallel framework.