Before anything is computed per pixel, the land surface is divided. The basin mask assigns every one of the 22.26 billion land pixels to the outlet it drains to: 24,587,290 outlets, 24,577,574 on the coast and 9,716 inland sinks, each with its catchment, from the Amazon down to single coastal pixels. The region mask groups whole basins into 90 regions, each small enough to process and to download on its own; no basin is cut by a region edge. Version 1.1 adds a third way of looking at the same basins: basin groups along the HydroBASINS Level-01, Level-02 and Level-03 units, 34, 86 and 316 groups. The Arctic (8) is kept as a Level-01 group of its own, and groups that carry no HydroBASINS code are kept as they are. A group has no size limit and nothing is computed on it. A region is a whole HydroBASINS Level-02 unit where it fits the capacity, and a run of its Level-03 units where it does not. These masks are the first layers in the release, and every later step works one region at a time. Pick a region on the map for its FullBasin files, the MERIT Hydro 5°×5° tiles it was computed from, and its window on the global 90 m grid. The MERIT Hydro layers are not redistributed here; you obtain them from their own archive under their terms.
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fullhydro.org