[GRASS-user] Algorithm in flow direction

[Markus Metz]

My first reply was maybe a bit short and rude, let me try again.

Both r.watershed and r.terraflow produce various output maps and it is 
not exactly clear to me what output you are interested in.
If you want flow accumulation, the threshold doesn't have an effect on 
it. Flow accumulation is calculated independent of such a threshold.
If you want rivers, you can either set the threshold in r.watershed and 
get stream segments or use the flow accumulation output of either module 
and do something like r.mapcalc "rivers = if(flow_acc > threshold, 1, 
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.
I'm not aware of a threshold option in r.terraflow. For r.watershed, 
there is no default value for threshold, you have to specify this 
threshold if you want stream segments, basins, or half basins, otherwise 
r.watershed terminates with an error.

Hope that helps more

Markus M


margherita wrote:
> Hi all
>
> i have some question about the commands r.watershed and r.terraflow. 
> By using both commands i obtained from a DEM a flow accumulation with 
> the algorithm of the single flow direction and a flow accumulation 
> with the algorithm of the multi flow direction. In both cases i did 
> not enter the treshold of cell (i.e. in the flow accumulation if a 
> grid cell value exceeds a certain threshold, it's part of the river, 
> if not, it isn't)
>
> Please can you let me know if there are default values of this 
> threshold and, if yes, what they are, and if that value can be changed 
> by the user and how.
>
> Thanks a lot.
>
> Regards
>