Flow direction

Dialog box

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.

Dialog box options:

Input DEM:

Select an input raster map. Open the list box and select the desired input map, or directly drag a raster map from a Catalog into this box.

The input map is supposed to be a sink-free Digital Elevation Model (DEM).

Steepest slope:

Select this option when flow direction should be calculated according to the steepest downhill slope between a central pixel and its 8 neighbour pixels.

Lowest height:

Select this option when flow direction should be simply be calculated as:

  • check whether any of the 8 neighbour pixels has a value smaller than the central pixel;
  • if so, then use the position of the pixel with the smallest pixel value to determine the flow direction for the central pixel.

Output raster map:

Type a name for the output raster map that will contain the flow directions.

Description:

Optionally, type a description for the output map. The description will appear in the status bar of the Main window when moving the mouse pointer over the map in a Catalog, and in the title bar of a map window when the output map is displayed. If no description is supplied, the output map will use its own definition as description.

When you click the Show button, the dependent output map will be defined, calculated and shown. When you click the Define button, the dependent output map will only be defined; if necessary the map will be calculated later, for instance when the map is opened to be displayed.

The output map will always use system domain FlowDirection.

See also: