RE: [GRASS-user] Use of r.watershed

GRASS GRASS user
1

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@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@insightbb.com, 309-255-2727
http://chuck.ehlschlaeger.info/

-----Original Message-----
From: grass-user-bounces@lists.osgeo.org
[mailto:grass-user-bounces@lists.osgeo.org] On Behalf Of
hydro@hydromodeling.com
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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
http://chuck.ehlschlaeger.info/

-----Original Message-----
From: grass-user-bounces@lists.osgeo.org
[mailto:grass-user-bounces@lists.osgeo.org] On Behalf Of
hydro@hydromodeling.com
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.
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2

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, <hydro@hydromodeling.com> 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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
http://chuck.ehlschlaeger.info/

-----Original Message-----
From: grass-user-bounces@lists.osgeo.org
[mailto: grass-user-bounces@lists.osgeo.org] On Behalf Of
hydro@hydromodeling.com

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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
http://chuck.ehlschlaeger.info/

-----Original Message-----
From: grass-user-bounces@lists.osgeo.org
[mailto: grass-user-bounces@lists.osgeo.org] On Behalf Of
hydro@hydromodeling.com
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
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3

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 <mseibel@gmail.com>:

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, <hydro@hydromodeling.com> 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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
> http://chuck.ehlschlaeger.info/
>
> -----Original Message-----
> From: grass-user-bounces@lists.osgeo.org
> [mailto:grass-user-bounces@lists.osgeo.org] On Behalf Of
> hydro@hydromodeling.com
> 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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
>> http://chuck.ehlschlaeger.info/
>>
>> -----Original Message-----
>> From: grass-user-bounces@lists.osgeo.org
>> [mailto:grass-user-bounces@lists.osgeo.org] On Behalf Of
>> hydro@hydromodeling.com
>> 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
>

_______________________________________________
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4

On Wednesday 23 January 2008, hydro@hydromodeling.com wrote:

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.

Here is a small comparison I did a while back on watershed delineation,
comparing r.watershed, r.terraflow, and basin from Arc/Info:

http://casoilresource.lawr.ucdavis.edu/drupal/node/284

note that it is a little sparse on the details, and could use some better
figures. see code examples for explanation.

cheers,

Dylan

Quoting M S <mseibel@gmail.com>:
> 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.h
>tml
>
>
> Mark
>
> On Jan 23, 2008 11:42 AM, <hydro@hydromodeling.com> 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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
>> > http://chuck.ehlschlaeger.info/
>> >
>> >
>> > -----Original Message-----
>> > From: grass-user-bounces@lists.osgeo.org
>> > [mailto:grass-user-bounces@lists.osgeo.org] On Behalf Of
>> > hydro@hydromodeling.com
>> > 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 <c.ehlschlaeger@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@insightbb.com, 309-255-2727
>> >> http://chuck.ehlschlaeger.info/
>> >>
>> >>
>> >> -----Original Message-----
>> >> From: grass-user-bounces@lists.osgeo.org
>> >> [mailto:grass-user-bounces@lists.osgeo.org] On Behalf Of
>> >> hydro@hydromodeling.com
>> >> 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.
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>>
>> 1/22/2008
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>> > 8:12 PM
>> >
>> >
>> > No virus found in this outgoing message.
>> > Checked by AVG Free Edition.
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>>
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>> _______________________________________________
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_______________________________________________
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--
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341

5

On Jan 23, 2008 6:28 PM, <hydro@hydromodeling.com> wrote:

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.

see also here for some comparison:
http://www.cs.duke.edu/geo*/terraflow/speedup.html

Markus

6

hydromodeling.com wrote:

> I have also tried using the -m and -4 flags placed after the
> r.watershed part.

On modern systems using -m will slow it down a lot for minor memory
savings. Don't use it. I wouldn't bother with -4 either.

r.terraflow

note as far as output basins go this calculates something different-
sink-watersheds not catchment watersheds. see
  http://thread.gmane.org/gmane.comp.gis.grass.user/21564/focus=21579

But r.terraflow should run in minutes.

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.

Yes I have run on something that size, and bigger. It could take up to
two days to complete, but it gets there with nice results. I usually
set it up to run on a friday afternoon and it's done by the time I get
back on monday. Setting the process priority low (ie high renice value)
helps if you leave it running during the week.

To get started, you might try the examples given for the Spearfish
dataset in the help page and on the wiki,

http://download.osgeo.org/grass/grass63/manuals/html63_user/r.watershed.html
  (help page home is in transition- broken link right now)
  http://grass.gdf-hannover.de/wiki/Psmap_flooding_example

Hamish

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7

Markus:

see also here for some comparison:
http://www.cs.duke.edu/geo*/terraflow/speedup.html

it doesn't list GRASS version but from the Pentium III and March 2001
references I would expect that the version of r.watershed there was
from GRASS 5.0 and using the old color setting code which proved to be
a terrible bottleneck. The r.watershed in newer versions of GRASS can
be much much faster.

Hamish

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