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Hello Pankaj<br>
(Posting back to the list also)<br>
<br>
<br>
On 24/11/2011 14:01, Pankaj Kr Sharma wrote:
<blockquote
cite="mid:CAM5_u0FgWa8rfvrZAQweqWvMK=m6LvQo21POKyK33hfWNeHV9Q@mail.gmail.com"
type="cite">
<pre wrap="">Hi,
Delay is sincerely regretted.
You can find the outputs below.
The result of r.report is attached separately.
(Thu Nov 24 17:07:28
2011)
r.watershed elevation=ybdem@work threshold=12345</pre>
</blockquote>
<br>
<br>
I see from your g.region output that the region resolution is 3
arcsecs. That's about 90m. So each cell is 8100 sq m. By choosing a
threshold of 12345, as above, you're asking for catchments with a
minimum size of 0.0081 X 12345 ~= 100 sq km.<br>
If you wanted a minimum of 300 sq km, then your threshold should be
about 37,000.<br>
( I hope I'm not repeating things you already know :-) )<br>
<br>
Looking thru the r.report file, many of the categories are > 100
sq km, matching your threshold value of 12345. There are also many
categories with a small total cell area near 0.0. These might be
areas near the edge of the region which flow out of the region, thus
are not part of any large basin.<br>
<br>
Another thing that surprised me, In your attached r.report file, I
saw that over 600,000 sq km were left with nodata? Why would that
be? Perhaps the DEM doesn't cover the whole region?<br>
r.info ybdem@work ?<br>
<br>
Regards,<br>
Micha<br>
<br>
<blockquote
cite="mid:CAM5_u0FgWa8rfvrZAQweqWvMK=m6LvQo21POKyK33hfWNeHV9Q@mail.gmail.com"
type="cite">
<pre wrap="">
accumulation=ybdem100sqkm_acc drainage=ybdem100sqkm_fdir
basin=ybdem100sqkm_basin stream=ybdem100sqkm_stream
SECTION 1a (of 5): Initiating Memory.
SECTION 1b (of 5): Determining Offmap Flow.
SECTION 2: A* Search.
SECTION 3: Accumulating Surface Flow with MFD.
SECTION 4: Watershed determination.
SECTION 5: Closing Maps.
(Thu Nov 24 17:09:38 2011) Command finished (130 sec)
(Thu Nov 24 17:28:15
2011)
r.info -g map=ybdem100sqkm_basin@work
north=32.0008205330556
south=21.9999912
east=82.0008005330556
west=72.9999708
(Thu Nov 24 17:28:15 2011) Command finished (0
sec)
(Thu Nov 24 17:28:52
2011)
g.region
-p
projection: 3 (Latitude-Longitude)
zone: 0
datum: wgs84
ellipsoid: wgs84
north: 32:00:02.953919N
south: 21:59:59.96832N
west: 72:59:59.89488E
east: 82:00:02.881919E
nsres: 0:00:02.999999
ewres: 0:00:02.999999
rows: 12001
cols: 10801
cells: 129622801
(Thu Nov 24 17:28:52 2011) Command finished (0
sec)
On Sun, Nov 20, 2011 at 4:25 PM, Micha Silver <a class="moz-txt-link-rfc2396E" href="mailto:micha@arava.co.il"><micha@arava.co.il></a> wrote:
</pre>
<blockquote type="cite">
<pre wrap=""> On 20/11/2011 12:50, Pankaj Kr Sharma wrote:
Dear Micha,
by the method of threshold value, I have tried and output varies from 1
sqkm to 6000 sqkm.
Not what I want.
Give us some more information:
g.region -p
r.info <your dem raster>
the exact command you used to run r.watersheds
On Sun, Nov 20, 2011 at 2:44 PM, Micha Silver <a class="moz-txt-link-rfc2396E" href="mailto:micha@arava.co.il"><micha@arava.co.il></a> wrote:
</pre>
<blockquote type="cite">
<pre wrap=""> On 20/11/2011 08:58, Pankaj Kr Sharma wrote:
Dear grass users,
I am trying to find sub-basins and their outlets exactly for each 300 sq.
km. in a bigger basin.
I think accumulation value indicates the no. of cells draining at a cell.
And will thus be a good starting point for such calculation.
The threshold value is what you're looking for. You need to calculate
how many raster cells it will take to make 300 sq. km - depending on the
raster resolution. For example, if the resolution is 90 m. (i.e. SRTM
elevation data) then each cell = 8100 sq.m. then you will need a threshold
of 370,000 to create basins of about 300 sq.km.
The other option is to start backwards from the basin outlet and keep
on investigating upwards in a stream network.
Can someone provide some hint / help in this regard.
Thanks.
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</pre>
</blockquote>
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