[GRASS-dev] r.watershed question

Markus Metz markus.metz.giswork at googlemail.com
Sat Dec 20 10:38:18 EST 2008



Michael Barton wrote:
> Thanks for the information. I did look through the docs but didn't see 
> this. I guess I missed it.
It is mentioned under OPTIONS, accumulation, at least in grass64 r34951 
(as of today).
>
> So...
>
> r.mapcalc positive_accum=if(accum_map<0,accum_map*-1,accum_map)
>
> ...would produce a normal flow accumulation map, but one that 
> underestimates flow in some areas?
Exactly. If you want accurate flow accumulation for a given area (at 
least as accurate as the input elevation allows), you have to increase 
the region until no more surface runoff from outside the region occurs 
for that area of interest. The basins connected to that area would then 
not touch the borders of the region.

Markus M
>
>
> Michael
> ____________________
> C. Michael Barton, Professor of Anthropology
> Director of Graduate Studies
> School of Human Evolution & Social Change
> Center for Social Dynamics & Complexity
> Arizona State University
>
> Phone: 480-965-6262
> Fax: 480-965-7671
> www: <www.public.asu.edu/~cmbarton>
>
>
>
> On Dec 20, 2008, at 1:59 AM, Markus Metz wrote:
>
>>
>>
>> Michael Barton wrote:
>>> Should r.watershed be giving negative accumulation values?
>> Yes. Negative accumulation values indicate that these cells receive 
>> offmap inflow or inflow from a MASKed area from somewhere upstream. 
>> This has always been the case for flow accumulation output. Negative 
>> flow accumulation can be interpreted as "at least so many cells drain 
>> into this cell, but probably more cells that were not included in the 
>> region/unMASKed area". State that more clearly in the manpage?
>>
>> Regards,
>>
>> Markus M
>
>


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