Hi,
when providing a binary depression layer for r.watershed, areas around
those depressions will be left out in the resulting basin map. Are
those areas the watersheds of the given depressions?
According to Markus Neteler's book "Open Source GIS", r.watershed
"does not require filling of depressions in DEM prior to it's
application [...]". What will be the difference if I use a
depressionless DEM (with/without binary depression layer)?
Thank's for your help. I'm new to this field so please excuse any
missing base knowledge!
Best regards,
Georg Kaspar
In short, r.watershed, without depression input, will route water in and up and out of depressions in the terrain to illustrate the complete downward path. This is why no DEM filling is necessary. By entering in known (substantial and impactful) depressions, the water does not route out of it.
The known depressions are especially useful when delineating sink-watersheds or internally drained, but can play a role in depressions where water doesnt flow out of the depression.
Mark
On Thu, Jan 29, 2009 at 10:12 AM, Georg Kaspar <georg@geofs.de> wrote:
Hi,
when providing a binary depression layer for r.watershed, areas around
those depressions will be left out in the resulting basin map. Are
those areas the watersheds of the given depressions?
According to Markus Neteler’s book “Open Source GIS”, r.watershed
“does not require filling of depressions in DEM prior to it’s
application […]”. What will be the difference if I use a
depressionless DEM (with/without binary depression layer)?
Thank’s for your help. I’m new to this field so please excuse any
missing base knowledge!
Best regards,
Georg Kaspar
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
Georg Kaspar wrote:
Hi,
when providing a binary depression layer for r.watershed, areas around
those depressions will be left out in the resulting basin map.
Maybe your basin threshold was too high, try with a lower value. Basins are only calculated if there is a stream exceeding the threshold.
Are
those areas the watersheds of the given depressions?
According to Markus Neteler's book "Open Source GIS", r.watershed
"does not require filling of depressions in DEM prior to it's
application [...]". What will be the difference if I use a
depressionless DEM (with/without binary depression layer)?
A depressionsless DEM is a DEM filled with r.fill.dir. Water will flow through where there was once a depression, i.e. in and out.
A binary depression layer as additional input to r.watershed indicates real depressions, like Mark said, water will flow into these depressions but not out. This is a substantial difference to a depressionless DEM.
Markus M
Thank's for your help. I'm new to this field so please excuse any
missing base knowledge!
Best regards,
Georg Kaspar
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
On Thu, 29 Jan 2009 10:20:36 -0500, M S wrote:
In short, r.watershed, without depression input, will route water in
and *up* and out of depressions in the terrain to illustrate the
complete downward path. This is why no DEM filling is necessary. By
entering in known (substantial and impactful) depressions, the water
does not route out of it.
But in my case, providing known depressions leads to areas of null()s
around those depressions (lakes by the way).
Since a lake stores water, it sounds reasonable to consider it a depression. That is what I used on an internally drained watershed, and it worked well.
More or less a feature you determine will “hold water”, and not overland flow. The other areas of all have flow characteristics (e.g. accumulation,direction).
Mark
On Thu, Jan 29, 2009 at 10:49 AM, Georg Kaspar <georg@geofs.de> wrote:
On Thu, 29 Jan 2009 10:20:36 -0500, M S wrote:
In short, r.watershed, without depression input, will route water in
and up and out of depressions in the terrain to illustrate the
complete downward path. This is why no DEM filling is necessary. By
entering in known (substantial and impactful) depressions, the water
does not route out of it.But in my case, providing known depressions leads to areas of null()s
around those depressions (lakes by the way).
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
Georg Kaspar wrote:
On Thu, 29 Jan 2009 10:20:36 -0500, M S wrote:
In short, r.watershed, without depression input, will route water in
and *up* and out of depressions in the terrain to illustrate the
complete downward path. This is why no DEM filling is necessary. By
entering in known (substantial and impactful) depressions, the water
does not route out of it.
But in my case, providing known depressions leads to areas of null()s around those depressions (lakes by the way).
If these lakes have an outflow, i.e. water is leaving these lakes, the results will be more realistic when you omit the depression input to r.watershed and only use the (not filled) DEM.
If you are talking about the basins output having NULL values around these depression, this is because your basin threshold value was too high, set it to a lower value. Check the current flow accumulation output for a reasonable threshold.
If these lakes have an outflow, i.e. water is leaving these lakes, the
results will be more realistic when you omit the depression input to
r.watershed and only use the (not filled) DEM.If you are talking about the basins output having NULL values around
these depression, this is because your basin threshold value was too
high, set it to a lower value. Check the current flow accumulation
output for a reasonable threshold.
so, my accumulation map contains values from -144714 to 58920 with a
majority of cells between 0-100. what would be an appropriate threshold
value? I already tried 5000 and it looked similar to the output I
received from r.terraflow, but when running r.watershed with depression
input I still receive those null()-areas...
by the way, this is what my region looks like:
GRASS 6.2.3 (Schweiz):~ > g.region -p
projection: 99 (Swiss. Obl. Mercator)
zone: 0
datum: ch1903
ellipsoid: bessel
north: 193996.9
south: 182009.4
west: 777510.9
east: 812479.6
nsres: 29.96875
ewres: 29.99030875
rows: 400
cols: 1166
cells: 466400
Georg Kaspar wrote:
If these lakes have an outflow, i.e. water is leaving these lakes, the
results will be more realistic when you omit the depression input to
r.watershed and only use the (not filled) DEM.If you are talking about the basins output having NULL values around
these depression, this is because your basin threshold value was too
high, set it to a lower value. Check the current flow accumulation
output for a reasonable threshold.
so, my accumulation map contains values from -144714 to 58920 with a majority of cells between 0-100. what would be an appropriate threshold value?
You can try standard deviation of flow accumulation (r.univar -g).
I already tried 5000 and it looked similar to the output I received from r.terraflow,
The basins of r.watershed look similar to the basins of r.terraflow?
but when running r.watershed with depression input I still receive those null()-areas...
... in the basins output I assume. If you really want to treat lakes as real depressions, i.e. water is not supposed to leave these lakes, then there will always be NULL areas around the lakes unless you set the threshold to something much smaller than 100, but then the stream segments and basins become meaningless because there are too many...
by the way, this is what my region looks like:
GRASS 6.2.3
Rather use 6.4.0RC3 instead of 6.2.3, you should get a couple of nice surprises...
[...]
cells: 466400
Not that many cells, should take just a few seconds to get the basins (with 6.4.0RC3).
R.terraflow outputs catchments around sinks in the dem. R.watershed should be making huge delineations in comparison.
Mark
On Feb 2, 2009, at 11:40 AM, Markus Metz <markus.metz.giswork@googlemail.com > wrote:
Georg Kaspar wrote:
If these lakes have an outflow, i.e. water is leaving these lakes, the
results will be more realistic when you omit the depression input to
r.watershed and only use the (not filled) DEM.If you are talking about the basins output having NULL values around
these depression, this is because your basin threshold value was too
high, set it to a lower value. Check the current flow accumulation
output for a reasonable threshold.so, my accumulation map contains values from -144714 to 58920 with a majority of cells between 0-100. what would be an appropriate threshold value?
You can try standard deviation of flow accumulation (r.univar -g).
I already tried 5000 and it looked similar to the output I received from r.terraflow,
The basins of r.watershed look similar to the basins of r.terraflow?
but when running r.watershed with depression input I still receive those null()-areas...
... in the basins output I assume. If you really want to treat lakes as real depressions, i.e. water is not supposed to leave these lakes, then there will always be NULL areas around the lakes unless you set the threshold to something much smaller than 100, but then the stream segments and basins become meaningless because there are too many...
by the way, this is what my region looks like:
GRASS 6.2.3
Rather use 6.4.0RC3 instead of 6.2.3, you should get a couple of nice surprises...
[...]
cells: 466400Not that many cells, should take just a few seconds to get the basins (with 6.4.0RC3).
_______________________________________________
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grass-user@lists.osgeo.org
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... in the basins output I assume. If you really want to treat lakes as
real depressions, i.e. water is not supposed to leave these lakes, then
there will always be NULL areas around the lakes unless you set the
threshold to something much smaller than 100, but then the stream
segments and basins become meaningless because there are too many...
ok, so I only need to specify depressions that do not loose water!?
Artificial depressions like postholes or pits for example!? What would be
the difference between purging the DEM from those depressions and
providing them as binary input for r.watershed?
Georg Kaspar wrote:
ok, so I only need to specify depressions that do not loose water!? Artificial depressions like postholes or pits for example!?
Rather something with a few hectares in size (at least), e.g. water reservoirs where water is pumped out, no overground outflow. Postholes and small pits are not able to distort r.watershed. The best example for a real depression is the Dead Sea, a big lake with no outflow.
What would be the difference between purging the DEM from those depressions and providing them as binary input for r.watershed?
Generally there is no need to purge the input DEM from depressions, why do you insist on doing so?
The difference between purging a DEM and providing an option depression=<map with real depressions> has been explained before in this thread [1], [2].
[1] http://lists.osgeo.org/pipermail/grass-user/2009-January/048604.html
[2] http://lists.osgeo.org/pipermail/grass-user/2009-January/048605.html
On Mon, 02 Feb 2009 19:31:06 +0100 Markus Metz wrote:
Generally there is no need to purge the input DEM from depressions, why
do you insist on doing so?
The difference between purging a DEM and providing an option
depression=<map with real depressions> has been explained before in this
thread [1], [2].
sorry, the whole thing just makes me crazy. I have never done this kind
of calculation but I need to do this right know in order to "transfer"
excercises written for ArcGIS to GRASS (and in ArcGIS you need to purge
those depressions...).
So far, I found out that I could bring up a proper solution for any
excercise using GRASS, but this whole watershed-thing keeps bothering
me 
Please excuse me repeating the same questions over and over again -
I am also new to newsgroups
On Mon, Feb 2, 2009 at 8:41 PM, Georg Kaspar <georg@geofs.de> wrote:
On Mon, 02 Feb 2009 19:31:06 +0100 Markus Metz wrote:
Generally there is no need to purge the input DEM from depressions, why
do you insist on doing so?
The difference between purging a DEM and providing an option
depression=<map with real depressions> has been explained before in this
thread [1], [2].sorry, the whole thing just makes me crazy. I have never done this kind
of calculation but I need to do this right know in order to "transfer"
excercises written for ArcGIS to GRASS (and in ArcGIS you need to purge
those depressions...).
So far, I found out that I could bring up a proper solution for any
excercise using GRASS, but this whole watershed-thing keeps bothering
me
I am also new to newsgroups
All fine and welcome! Please collect relevant hints in the
GRASS Wiki (after registration pages can be edited or created).
Thanks
Markus