[GRASS-user] r.watershed negative values in flow accumulation
jarekj at amu.edu.pl
Mon Jul 27 13:48:50 EDT 2009
it ma happen if you area is middle part of extensive cachment
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
>> BTW it is very good idea to use negative values for that cells, As
>> far as I know only grass had that feature
> 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.
>>> 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.
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