[GRASS-user] r.watershed negative values in flow accumulation

Margherita Di Leo diregola at gmail.com
Mon Jul 27 08:36:44 EDT 2009 

Jarek Jasiewicz wrote:
>
>
> Margherita Di Leo pisze:
>> Hi all,
>>
>> I found a strange result runnig r.watershed . There are _many_ 
>> negative values in flow accumulation obtained with Multiple Flow 
>> Direction algorithm (MFD, f flag). I compared with the accumulation 
>> map obtained with r.terraflow. It is surprising to see that with 
>> r.terraflow there are very few negative values. How is it possible? 
>> Is the algorithm (MFD) the same? How is the true result?
>>
>> I'm runnig Grass 6.5 svn on Ubuntu 8.04 - 64 bit.
> negative values are ascribed to that cells where real cumulated flow 
> cannot be calculated. In most cases it is for streams which drain from 
> outside area. See help for r.watershed, it is well described there.
> If you want all values positve run: echo 'new_accum=abs(accum)'|r.mapcalc
> BTW it is very good idea to use negative values for that cells, As far 
> as I know only grass had that feature
> Jarek
Thanks for your answer.
The problem is to understand the fact that almost every values are 
negative, and it is a very big region as you can see:

GRASS 6.5.svn (Arno):~ > g.region -p
projection: 1 (UTM)
zone:       32
datum:      wgs84
ellipsoid:  wgs84
north:      4891280
south:      4756620
west:       596320
east:       759480
nsres:      20
ewres:      20
rows:       6733
cols:       8158
cells:      54927814

attached the results of r.stats for r.watershed and r.terraflow. In the 
map obtained by r.terraflow there are negative values too, but are few, 
so it seems to me more realistic.

Cheers

margherita

>>
>> Command lines:
>>
>> r.watershed -f elevation=elev1 at ex_20 accumulation=accumulation_MFD 
>> convergence=5 memory=300
>>
>> r.terraflow elevation=elev1 at ex_20 filled=filled_terraflow 
>> direction=dir_terraflow swatershed=swa_terraflow 
>> accumulation=acc_terraflow tci=tci d8cut=infinity memory=300 
>> STREAM_DIR=/var/tmp stats=/home/margherita/terraflow_stats.out
>>
>> Thank you in advance.
>>
>> margherita
>>
>


-- 
Eng. Margherita Di Leo
Ph.D. Student 
Methods and Technologies for Environmental Monitoring
Department of Environmental Engineering and Physics (DIFA)

University of Basilicata 
Campus Macchia Romana
85100 - Potenza 
Italy

Office: +39-0971205363
Fax: +39-0971205160
E-mail: dileomargherita AT gmail DOT com
Skype: dileomargherita
URL: http://www.difa.unibas.it/A_Manager_PP.do?azione=visualizzaHomePage&id=106





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