Drainage network extraction

Functionality

The Drainage Network Extraction operation extracts a basic drainage network (boolean raster map). The output raster map will show the basic drainage as pixels with value True, while other pixels have value False.

As input is required: the output raster map of the Flow Accumulation operation. This map contains a cumulative drainage count for each pixel.

Subsequently, you can choose to use either:

a threshold value, i.e. a value for the minimum number of pixels that are supposed to drain into a pixel to let this pixel remain as a drainage in the output map; the larger you choose this value, the fewer drainages will remain in the output map;
a threshold raster map that contains variable threshold values. A threshold map can for instance be based on geological units, on height values, or on an internal relief map, etc.

when using a threshold map, also the output map of a previous Flow direction operation is required. The program then uses the flow direction map to automatically fill possible gaps between extracted drainage lines.

Depending on the flow accumulation value for a pixel and the threshold value for this pixel, it is decided whether true or false should be assigned to the output pixel. If the flow accumulation value of a pixel exceeds the threshold value, the output pixel value will be true; else, false is assigned.

For instance, when a stream threshold value of 1000 is used:

if a pixel in the flow accumulation map has a value > 1000, this pixel will be assigned value True in the output drainage network map;
else value False will be assigned to the output pixel.

Tips:

A threshold map can be prepared with the Variable threshold computation operation. A threshold map may be based on internal relief values or on specific infiltration values for specific geological units, etc.
When using a threshold map, the operation will furthermore fill possible gaps between extracted drainage lines.

Example:

Input flow accumulation map:	Output drainage network map (threshold 1000):	Output drainage network map (variable threshold):

Larger values in red, and smaller values in blue.	True values in green, and False values in gray.	True values in green, and False values in gray.

Input map requirements:

The required input map is a raster map produced by the Flow accumulation operation, i.e. a raster map using system domain Value.
Optionally, you can use a raster map containing threshold values; this map should thus be a value map.
When using such a threshold map, you also need to specify a Flow direction map. This map is used to automatically fill possible gaps between extracted drainage lines. A flow direction map always uses system domain FlowDirection.

Domain and georeference of output map:

The ouput map will use system domain Bool; the output map will use the same georeference as the input map(s).

Tip:

The output map of the Drainage network extraction operation can be used as input for the Drainage network ordering operation.