Computing the drainage network on huge grid terrains

Abstract

We present a very efficient algorithm, named EMFlow, and its implementation to compute the drainage network, that is, the flow direction and flow accumulation on huge terrains stored in external memory. It is about 20 times faster than the two most recent and most efficient published methods: TerraFlow and r.watershed.seg. Since processing large datasets can take hours, this improvement is very significant. The EMFlow is based on our previous method RWFlood which uses a flooding process to compute the drainage network. And, to reduce the total number of I/O operations, EMFlow is based on grouping the terrain cells into blocks which are stored in a special data structure managed as a cache memory. Also, a new strategy is adopted to subdivide the terrains in islands which are processed separately. Because of the recent increase in the volume of high resolution terrestrial data, the internal memory algorithms do not run well on most computers and, thus, optimizing the massive data processing algorithm simultaneously for data movement and computation has been a challenge for GIS.