[GRASS-user] trace channel from accummulation raster

[Markus Metz]

Christoff wrote:
> Hi everybody,
>
> Is there a way to extract a channels from the accumulation raster, e.g.
> accumulation from r.watershed?
>   
You can either set the threshold option in r.watershed and get stream 
segments or use flow accumulation output and do something like r.mapcalc 
"channels = if(flow_acc > threshold, flow_acc, null())", threshold would 
here be the desired number of cells draining through a given cell when 
using r.watershed flow accumulation or size of the area draining through 
a given cell for r.terraflow flow accumulation.
See also "Further processing of output layers" in the manual of r.watershed.
> Somehow r.drain delivers something which is not following the accumulation
> raster. In theory the accumulation along the drainage shoult always increase
> and never decrease. My results show a nice increasing trend, which is
> interrupted with very low values.
>   
As I understand, r.drain works on elevation and uses a different 
algorithm, so the results are likely different, but I don't know r.drain 
that well.