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: grass-user@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 <[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: grass-user@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. > >> _______________________________________________ > >> grass-user mailing list > >> grass-user@lists.osgeo.org > >> http://lists.osgeo.org/mailman/listinfo/grass-user > >> > >> No virus found in this incoming message. > >> Checked by AVG Free Edition. > >> Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: > 1/22/2008 > >> 8:12 PM > >> > >> > >> No virus found in this outgoing message. > >> Checked by AVG Free Edition. > >> Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: > 1/22/2008 > >> 8:12 PM > >> > >> > > > > > > _______________________________________________ > > grass-user mailing list > > grass-user@lists.osgeo.org > > http://lists.osgeo.org/mailman/listinfo/grass-user > > > > No virus found in this incoming message. > > Checked by AVG Free Edition. > > Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: > 1/22/2008 > > 8:12 PM > > > > > > No virus found in this outgoing message. > > Checked by AVG Free Edition. > > Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: > 1/22/2008 > > 8:12 PM > > > > > > > _______________________________________________ > grass-user mailing list > grass-user@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/grass-user >
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