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

[Jarek Jasiewicz]

it ma happen if you area is middle part of extensive cachment
Jarek

Margherita Di Leo pisze:
> 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
>>>
>>
>
>