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.
<br><br>From the r.watershed manual, "If watershed divides contain flat to low slope, <em>r.watershed</em>
will generate better basin results than <em>r.terraflow</em>.
(<em>r.terraflow</em> uses the same type of algorithm as ESRI's ArcGIS
watershed software which fails under these conditions.)" <a href="http://download.osgeo.org/grass/grass63/manuals/html63_user/r.watershed.html">http://download.osgeo.org/grass/grass63/manuals/html63_user/r.watershed.html
</a><br><br><br>Mark<br><br><div class="gmail_quote">On Jan 23, 2008 11:42 AM, <<a href="mailto:hydro@hydromodeling.com">hydro@hydromodeling.com</a>> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Thanks. I will try and change my threshold value this evening.<br><br>A little background, I have completed this phase of the project at<br>"work" using ArcHydro and it took about half a day total to run<br>through the tools (fill, flow direction, flow accumulation, etc.). I
<br>am trying to now do the same project at home using open source tools<br>as a means of learning grass and as a comparison. I wonder if the<br>r.watershed algorithms differ enough from the ArcHydro ones to make<br>execution times very different? Any thoughts?
<br><div class="Ih2E3d"><br>Quoting Charles Ehlschlaeger <<a href="mailto:c.ehlschlaeger@insightbb.com">c.ehlschlaeger@insightbb.com</a>>:<br><br></div>> r.watershed will take a long time for a 8,600x10,300 cell DEM under any
<br>> circumstance. Your threshold value (400) might also be slowing things down.<br>><br>> How large should your smallest exterior watershed basin be? If your grid<br>> cells are 10mx10m, your basins will be 400x10mx10m = 40,000 m^2: not that
<br>> large. The larger the th value, the faster the program will run.<br><div class="Ih2E3d">><br>> Sincerely, chuck<br>><br>> Chuck Ehlschlaeger, Geospatial Science Consultant<br>> PERTAN GROUP, <a href="http://pertan.com/" target="_blank">
http://pertan.com/</a><br>> <a href="mailto:c.ehlschlaeger@insightbb.com">c.ehlschlaeger@insightbb.com</a>, 309-255-2727<br>> <a href="http://chuck.ehlschlaeger.info/" target="_blank">http://chuck.ehlschlaeger.info/
</a><br>><br>><br>> -----Original Message-----<br>> From: <a href="mailto:grass-user-bounces@lists.osgeo.org">grass-user-bounces@lists.osgeo.org</a><br>> [mailto:<a href="mailto:grass-user-bounces@lists.osgeo.org">
grass-user-bounces@lists.osgeo.org</a>] On Behalf Of<br>> <a href="mailto:hydro@hydromodeling.com">hydro@hydromodeling.com</a><br></div><div><div></div><div class="Wj3C7c">> Sent: Wednesday, January 23, 2008 8:52 AM
<br>> To: Charles Ehlschlaeger<br>> Cc: <a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a><br>> Subject: RE: [GRASS-user] Use of r.watershed<br>><br>> Chuck,<br>><br>> Thanks for your reply. The DEM has 8,627 rows and 10,270 columns.
<br>> The command line statement I used is:<br>><br>> r.watershed el=fill th=400 ac=fac ba=basins st=str<br>><br>> I have also tried using the -m and -4 flags placed after the r.watershed<br>> part.<br>>
<br>> Thanks,<br>><br>> Matt<br>><br>> Quoting Charles Ehlschlaeger <<a href="mailto:c.ehlschlaeger@insightbb.com">c.ehlschlaeger@insightbb.com</a>>:<br>><br>>> How many rows and columns are in the input map? Also include the exact
<br>>> command line statement you use to run the program.<br>>><br>>> Sincerely, chuck<br>>><br>>> Chuck Ehlschlaeger, Geospatial Science Consultant<br>>> PERTAN GROUP, <a href="http://pertan.com/" target="_blank">
http://pertan.com/</a><br>>> <a href="mailto:c.ehlschlaeger@insightbb.com">c.ehlschlaeger@insightbb.com</a>, 309-255-2727<br>>> <a href="http://chuck.ehlschlaeger.info/" target="_blank">http://chuck.ehlschlaeger.info/
</a><br>>><br>>><br>>> -----Original Message-----<br>>> From: <a href="mailto:grass-user-bounces@lists.osgeo.org">grass-user-bounces@lists.osgeo.org</a><br>>> [mailto:<a href="mailto:grass-user-bounces@lists.osgeo.org">
grass-user-bounces@lists.osgeo.org</a>] On Behalf Of<br>>> <a href="mailto:hydro@hydromodeling.com">hydro@hydromodeling.com</a><br>>> Sent: Wednesday, January 23, 2008 7:15 AM<br>>> To: <a href="mailto:grass-user@lists.osgeo.org">
grass-user@lists.osgeo.org</a><br>>> Subject: [GRASS-user] Use of r.watershed<br>>><br>>> Hello All,<br>>><br>>> I am new to GRASS but have been using GIS (mainly ESRI products) for<br>>> quite a few years. I am trying to learn how to use GRASS to carry out
<br>>> hydrology related analyses. Currently, I am trying to use the<br>>> r.watershed command to delineate the basins for a watershed. My<br>>> question is, how long would you expect it to take to run? I am
<br>>> wondering if I am doing it wrong or if it just takes longer than I<br>>> would expect. The DEM is about 170MB TIFF before I import it to<br>>> GRASS. I have tried to run it a few times using different options,
<br>>> each time letting it run for 3 - 4 hours before killing it. I have<br>>> tried both the -m and -4 flags. Any input would be appreciated.<br>>> _______________________________________________<br>>> grass-user mailing list
<br>>> <a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a><br>>> <a href="http://lists.osgeo.org/mailman/listinfo/grass-user" target="_blank">http://lists.osgeo.org/mailman/listinfo/grass-user
</a><br>>><br>>> No virus found in this incoming message.<br>>> Checked by AVG Free Edition.<br>>> Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: 1/22/2008<br>>> 8:12 PM<br>
>><br>>><br>>> No virus found in this outgoing message.<br>>> Checked by AVG Free Edition.<br>>> Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: 1/22/2008<br>>> 8:12 PM
<br>>><br>>><br>><br>><br>> _______________________________________________<br>> grass-user mailing list<br>> <a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a><br>> <a href="http://lists.osgeo.org/mailman/listinfo/grass-user" target="_blank">
http://lists.osgeo.org/mailman/listinfo/grass-user</a><br>><br>> No virus found in this incoming message.<br>> Checked by AVG Free Edition.<br>> Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: 1/22/2008
<br>> 8:12 PM<br>><br>><br>> No virus found in this outgoing message.<br>> Checked by AVG Free Edition.<br>> Version: 7.5.516 / Virus Database: 269.19.9/1238 - Release Date: 1/22/2008<br>> 8:12 PM<br>
><br>><br><br><br>_______________________________________________<br>grass-user mailing list<br><a href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a><br><a href="http://lists.osgeo.org/mailman/listinfo/grass-user" target="_blank">
http://lists.osgeo.org/mailman/listinfo/grass-user</a><br></div></div></blockquote></div><br>