Operation to compute a flow direction raster from an elevation raster
The directional encoding is from:
Greenlee,D. D. 1987. "Tile and Vector Processing for Scanned Linework."
Photogrammetric Engineering and Remote Sensing (ISSN 0099-1112),vol. 53,Oct. 1987,p. 1383-1387.
The direction of flow is towards the neighboring cell with the largest drop in elevation. If two
or more cells have the same drop in elevation,their directional values are added together. The
8-bit encoding of the direction preserves the multi-directional property.
Sinks, cells which have no drop in elevation towards any neighbor, have no direction of flow.
Determine flow direction of the cell at (c, r) in given raster
Produces a map of available immediate neighbors and their drop in elevation from the provided cell
Determines whether or not the cell at (c, r) in given raster is a sink