Flow direction

Algorithm

In a (sink-free) Digital Elevation Model (DEM), the Flow direction operation determines into which neighbouring pixel any water in a central pixel will flow naturally.

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

Steepest slope method:

 

Input values:   Height differences:     Distances:     Steepest slope:   Output flow direction:
  /   =    

 

Lowest height method:

 

Input values: Height differences: Largest height difference: Output flow direction:

 

Additional rules that are applied:

See also: