[GRASS-user] Re: IN: raster-flow-direction OUT:vector-network? (achim)

[Markus GRASS]

achim wrote:
> Thanks for reply,
>
> but thats not exactly what I am searching for. I can not to use the
> accumulation values, because
>
>
> i) I want to figure out the watersheds on parts of the network
> (beginning from certain points of the river network to separate some
> basins on different levels; 
r.water.outlet would do that, granted that flow direction has the same
format like r.watershed's flow direction
> ii) I am going to do some statistics on stream lengths /accumulations on
> network parts
>   
Is the suggestion by Margherita Di Leo to get streams not working?

> iii) I need to separate parallel flowing streams from each other which
> does not work on my current resolution
>   
I think v.generalize can separate parallel lines with method=displacement.
>
> PPS:
>   
>> r.mapcalc 'log_accumulation=log(abs(accumulation)+1)'
>>     
> QUESTION:
> why abs? accumulation values should be positive!?
>   
Depends on the module used to create flow accumulation. r.watershed
produces negative flow accumulation values for areas where flow
accumulation is a likely underestimate (basin extending beyond region
extends or parts of the basin are masked out). Also see r.watershed manual.

The mapcalc approach may not work well on MFD accumulation maps; there
the stream output of r.watershed would help.

Markus M