This drainage direction map for South America defines the direction of flow from each cell in the conditioned digital elevation model to its steepest downslope neighbor. Values of flow direction vary from 1 to 128. There are eight valid output directions relating to the eight adjacent cells into which flow could travel. This approach is commonly referred to as an eight-direction (D8) flow model and follows an approach presented in Jenson and Domingue (1988). All final outlet cells to the ocean are flagged with a value of 0. All cells that mark the lowest point of an endorheic basin (inland sink) are flagged with a value of -1. This data is derived from elevation data of the Shuttle Radar Topography Mission (SRTM) at 15 arc-second resolution approx. 500m at the equator.The goal of developing HydroSHEDS was to generate key data layers to support regional and global watershed analyses, hydrological modeling, and freshwater conservation planning at a quality, resolution and extent that had previously been unachievable.