[GRASS-user] Use of r.watershed

[hydro at hydromodeling.com]

Thanks.  I will try and change my threshold value this evening.

A little background, I have completed this phase of the project at  
"work" using ArcHydro and it took about half a day total to run  
through the tools (fill, flow direction, flow accumulation, etc.).  I  
am trying to now do the same project at home using open source tools  
as a means of learning grass and as a comparison.  I wonder if the  
r.watershed algorithms differ enough from the ArcHydro ones to make  
execution times very different?  Any thoughts?

Quoting Charles Ehlschlaeger <c.ehlschlaeger at insightbb.com>:

> r.watershed will take a long time for a 8,600x10,300 cell DEM under any
> circumstance. Your threshold value (400) might also be slowing things down.
>
> How large should your smallest exterior watershed basin be? If your grid
> cells are 10mx10m, your basins will be 400x10mx10m = 40,000 m^2: not that
> large. The larger the th value, the faster the program will run.
>
> Sincerely, chuck
>
> Chuck Ehlschlaeger, Geospatial Science Consultant
> PERTAN GROUP, http://pertan.com/
> c.ehlschlaeger at insightbb.com, 309-255-2727
> http://chuck.ehlschlaeger.info/
>
>
> -----Original Message-----
> From: grass-user-bounces at lists.osgeo.org
> [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of
> hydro at hydromodeling.com
> Sent: Wednesday, January 23, 2008 8:52 AM
> To: Charles Ehlschlaeger
> Cc: grass-user at lists.osgeo.org
> Subject: RE: [GRASS-user] Use of r.watershed
>
> Chuck,
>
> Thanks for your reply.  The DEM has 8,627 rows and 10,270 columns.
> The command line statement I used is:
>
> r.watershed el=fill th=400 ac=fac ba=basins st=str
>
> I have also tried using the -m and -4 flags placed after the r.watershed
> part.
>
> Thanks,
>
> Matt
>
> Quoting Charles Ehlschlaeger <c.ehlschlaeger at insightbb.com>:
>
>> How many rows and columns are in the input map? Also include the exact
>> command line statement you use to run the program.
>>
>> Sincerely, chuck
>>
>> Chuck Ehlschlaeger, Geospatial Science Consultant
>> PERTAN GROUP, http://pertan.com/
>> c.ehlschlaeger at insightbb.com, 309-255-2727
>> http://chuck.ehlschlaeger.info/
>>
>>
>> -----Original Message-----
>> From: grass-user-bounces at lists.osgeo.org
>> [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of
>> hydro at hydromodeling.com
>> Sent: Wednesday, January 23, 2008 7:15 AM
>> To: grass-user at lists.osgeo.org
>> Subject: [GRASS-user] Use of r.watershed
>>
>> Hello All,
>>
>> I am new to GRASS but have been using GIS (mainly ESRI products) for
>> quite a few years.  I am trying to learn how to use GRASS to carry out
>> hydrology related analyses.  Currently, I am trying to use the
>> r.watershed command to delineate the basins for a watershed.  My
>> question is, how long would you expect it to take to run?  I am
>> wondering if I am doing it wrong or if it just takes longer than I
>> would expect.  The DEM is about 170MB TIFF before I import it to
>> GRASS.  I have tried to run it a few times using different options,
>> each time letting it run for 3 - 4 hours before killing it. I have
>> tried both the -m and -4 flags.  Any input would be appreciated.
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>
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