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

Margherita Di Leo diregola at gmail.com
Mon Jul 27 07:24:34 EDT 2009

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.

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.


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 

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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