Thanks for pointing out the excerpt from the manual. So, based on
that it seems that r.watershed will take longer to run than the
ArcHydro tools. How much longer? I guess I will just let it run and
see if I can find out. Then I will run r.terraflow and compare the
differences between ArcHydro, r.terrafow and r.watershed.
If anyone has used r.watershed on a comparably sized DEM, I would be
interested to hear how long it took to run. By the way my machine has
a 2.8Ghz P4 processor and 1.25G of RAM.
Quoting M S <[EMAIL PROTECTED]>:
If you are in a relatively flat area, there needs to be consideration given
to the algorithm (to my knowledge there is no "choice" in the ArcGIS suite)
used to do the most important thing: delineate the watershed basin. Since
the question was asked about differences, this may apply if your terrain is
relatively flat.
From the r.watershed manual, "If watershed divides contain flat to low
slope, *r.watershed* will generate better basin results than *r.terraflow*.
(*r.terraflow* uses the same type of algorithm as ESRI's ArcGIS watershed
software which fails under these conditions.)"
http://download.osgeo.org/grass/grass63/manuals/html63_user/r.watershed.html
Mark
On Jan 23, 2008 11:42 AM, <[EMAIL PROTECTED]> wrote:
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 <[EMAIL PROTECTED]>:
> 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/
> [EMAIL PROTECTED], 309-255-2727
> http://chuck.ehlschlaeger.info/
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of
> [EMAIL PROTECTED]
> Sent: Wednesday, January 23, 2008 8:52 AM
> To: Charles Ehlschlaeger
> Cc: [email protected]
> 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 <[EMAIL PROTECTED]>:
>
>> 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/
>> [EMAIL PROTECTED], 309-255-2727
>> http://chuck.ehlschlaeger.info/
>>
>>
>> -----Original Message-----
>> From: [EMAIL PROTECTED]
>> [mailto:[EMAIL PROTECTED] On Behalf Of
>> [EMAIL PROTECTED]
>> Sent: Wednesday, January 23, 2008 7:15 AM
>> To: [email protected]
>> 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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