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.
You can choose whether you wish to calculate the flow direction for the central pixels:
When the position of the steepest-slope-neighbour pixel or the lowest-height-neighbour pixel is determined, the flow direction for the central pixel is known.
Example Steepest Slope method:
For each block of 3x3 input pixels, height differences are calculated between the central pixel and the 8 neighbours.
To compensate for distances:
Then, the neighbour with the largest downhill slope steepness value is found.
The position of this neighbour determines the flow direction of the current central pixel.
Input DEM | Calculating according to steepest slope | Output flow direction map |
![]() |
![]() |
![]() |
For more information and additional rules that are applied, see Flow direction : algorithm.
Example Lowest Height method:
For each block of 3x3 input pixels, height differences are calculated between the central pixel and the 8 neighbours.
Then, from the neighbours with values smaller than the current central pixel, the neighbour with the smallest value is found.
The position of this neighbour determines the flow direction of the current central pixel.
Input DEM | Calculating according to lowest height | Output flow direction map |
![]() |
![]() |
![]() |
For more information and additional rules that are applied, see Flow direction : algorithm.
Input map requirements:
The input map should be a value map; the input map is expected to be a sink-free Digital Elevation Model (DEM).
Domain and georeference of output map:
The output map will use system domain FlowDirection.
The output map will use the same georeference as the input map.
Tip:
The output map of the Flow direction operation can be used as input map for the Flow accumulation operation.
See also: