[GRASS-user] v.water.outlet

GRASS GRASS user
1

Hi all.
I tried this command, but I always get a map full of 0. I start from a drainage map from r.watershed, as described. Anyone confirms, or am I doing something wrong?
All the best.

--
Paolo Cavallini
See: http://www.faunalia.it/pc

2

I guess you mean r.water.outlet?

You probably know already, but maybe it’s worth repeating:

1- The input must be the drainage direction map from r.watershed.

2- And it’s crucial that the easting and northing parameters for the outlet point lay exactly on a stream. Zoom in very close to the streams raster at the outlet point and check for the exact coordinates.

··· 
-- 
Micha Silver
GIS Consultant, Arava Development Co.
[http://www.surfaces.co.il](http://www.surfaces.co.il)
3

Moreover,
check that your dem has sinks filled (r.fill.dir)

Bye,
Maxi

··· 
-- 
Micha Silver
GIS Consultant, Arava Development Co.
[http://www.surfaces.co.il](http://www.surfaces.co.il)


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-- 

Dr. Eng. Massimiliano Cannata
Responsabile Area Geomatica
Istituto Scienze della Terra
Scuola Universitaria Professionale della Svizzera Italiana
Via Trevano, c.p. 72
CH-6952 Canobbio-Lugano
Tel: +41 (0)58 666 62 14
Fax +41 (0)58 666 62 09
4

Il 04/01/2012 20:55, Micha Silver ha scritto:

I guess you mean r.water.outlet?

Yes, sorry.

1- The input must be the drainage *direction* map from r.watershed.

ok

2- And it's crucial that the easting and northing parameters for the outlet point lay
exactly on a stream. Zoom in very close to the streams raster at the outlet point and
check for the exact coordinates.

Checked, it seems ok.
The output maps is all 0s, with only one cell (close to the point I chose, but not on it) with value 1.

Thanks.

--
Paolo Cavallini - Faunalia
www.faunalia.eu
Full contact details at www.faunalia.eu/pc

5

On 05/01/12 12:16, Paolo Cavallini wrote:

Il 04/01/2012 20:55, Micha Silver ha scritto:

I guess you mean r.water.outlet?

Yes, sorry.

1- The input must be the drainage *direction* map from r.watershed.

ok

2- And it's crucial that the easting and northing parameters for the
outlet point lay
exactly on a stream. Zoom in very close to the streams raster at the
outlet point and
check for the exact coordinates.

Checked, it seems ok.
The output maps is all 0s, with only one cell (close to the point I
chose, but not on it) with value 1.

AFAIU r.water.outlet is extremely sensitive to local variations of terrain. Check the drainage direction map close to your point to see whether the resulting watershed is coherent with these directions.

I don't know if r.fill.dir on the DEM before calulating the drainage directions might help ?

Moritz

6

Il 05/01/2012 12:35, Moritz Lennert ha scritto:

AFAIU r.water.outlet is extremely sensitive to local variations of terrain. Check the
drainage direction map close to your point to see whether the resulting watershed is
coherent with these directions.

Oh, this could be an explanation - the original DEM is quite detailed (lidar).
Thanks a lot.

--
Paolo Cavallini - Faunalia
www.faunalia.eu
Full contact details at www.faunalia.eu/pc

7

On Thu, Jan 5, 2012 at 12:16 PM, Paolo Cavallini <cavallini@faunalia.it> wrote:

Il 04/01/2012 20:55, Micha Silver ha scritto:

I guess you mean r.water.outlet?

Yes, sorry.

1- The input must be the drainage *direction* map from r.watershed.

ok

2- And it's crucial that the easting and northing parameters for the
outlet point lay
exactly on a stream. Zoom in very close to the streams raster at the
outlet point and
check for the exact coordinates.

Checked, it seems ok.
The output maps is all 0s, with only one cell (close to the point I chose,
but not on it) with value 1.

This is weird. The cell in which the point you chose is located should
always be included in the basin created by r.water.outlet. Make sure
your computational region is set to the drainage raster. At least we
know now that r.water.outlet produces something, because the output is
not all 0s, at least one cell is 1.

Markus M

8

On Thu, Jan 5, 2012 at 12:12 PM, Massimiliano Cannata
<massimiliano.cannata@supsi.ch> wrote:

Moreover,
check that your dem has sinks filled (r.fill.dir)

Since the drainage map has been created by r.watershed, sink filling
is not only not needed but does more harm than good and should not be
used to prepare a DEM for r.watershed.

Markus M

On 01/04/2012 08:55 PM, Micha Silver wrote:

On 01/04/2012 07:27 PM, Paolo Cavallini wrote:

Hi all.
I tried this command, but I always get a map full of 0. I start from a
drainage map from r.watershed, as described. Anyone confirms, or am I doing
something wrong?
All the best.

I guess you mean r.water.outlet?

You probably know already, but maybe it's worth repeating:

1- The input must be the drainage *direction* map from r.watershed.

2- And it's crucial that the easting and northing parameters for the outlet
point lay exactly on a stream. Zoom in very close to the streams raster at
the outlet point and check for the exact coordinates.

--
Micha Silver
GIS Consultant, Arava Development Co.
http://www.surfaces.co.il

_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

--

Dr. Eng. Massimiliano Cannata
Responsabile Area Geomatica
Istituto Scienze della Terra
Scuola Universitaria Professionale della Svizzera Italiana
Via Trevano, c.p. 72
CH-6952 Canobbio-Lugano
Tel: +41 (0)58 666 62 14
Fax +41 (0)58 666 62 09

_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user

9

On 06/01/12 09:23, Markus Metz wrote:

On Thu, Jan 5, 2012 at 12:12 PM, Massimiliano Cannata
<massimiliano.cannata@supsi.ch> wrote:

Moreover,
check that your dem has sinks filled (r.fill.dir)

Since the drainage map has been created by r.watershed, sink filling
is not only not needed but does more harm than good and should not be
used to prepare a DEM for r.watershed.

for r.water.outlet I suppose...

Could you explain why ?

Moritz

10

On Fri, Jan 6, 2012 at 9:42 AM, Moritz Lennert
<mlennert@club.worldonline.be> wrote:

On 06/01/12 09:23, Markus Metz wrote:

On Thu, Jan 5, 2012 at 12:12 PM, Massimiliano Cannata
<massimiliano.cannata@supsi.ch> wrote:

Moreover,
check that your dem has sinks filled (r.fill.dir)

Since the drainage map has been created by r.watershed, sink filling
is not only not needed but does more harm than good and should not be
used to prepare a DEM for r.watershed.

for r.water.outlet I suppose...

Could you explain why ?

Because r.water.outlet generates a watershed basin from a drainage
direction map from r.watershed (says the manual), more generally,
r.water.outlet uses as input map only a drainage map and not a DEM,
thus it does not matter where the drainage map comes from, it could in
theory also be properly encoded moving directions from a cost surface
analysis. As pointed out by Micha and you, care must be taken with the
coordinates, otherwise an unexpectedly small basin will be delineated.
Additionally, r.water.outlet will most probably not work properly if
the current region does not match exactly the drainage map.

r.watershed has been designed from the very beginning by Chuck
Ehlschlaeger in 1989 to work with raw DEMs and to produce realistic
output without sink filling (see references in the manual).

Markus M