Flow direction

The Flow direction operation determines into which neighbouring pixel any water in a central pixel will flow.

Flow direction is calculated for every central pixel in input blocks of 3 by 3 pixels, each time comparing the value of the central pixel with the value of its 8 neighbouring pixels. The output map contains flow directions as N (to the North), NE (to the North East), etc.

You can choose to calculate flow direction according to steepest slope or according to lowest neighbour.