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On 09/11/2022 8:53, Alister Hood via
Qgis-user wrote:
Date: Wed, 9 Nov 2022
11:04:02 +1300
From: "Tony Shepherd \(FarmMaps NZ\)" <t...@farmmaps.nz>
To: "'Qgis Users List'" <qgis-user@lists.osgeo.org>
Subject: [Qgis-user] Watershed - Catchment area upstream of
a point
Message-ID: <003101d8f3be$04ed8380$0ec88a80$@farmmaps.nz>
Content-Type: text/plain; charset="us-ascii"
Hi All
I'm wanting to use a DEM/DTM to automatically map the
watershed/catchment
area upstream of a known point.
Yes, Grass is preferable because the integration with
QGIS works reliably. It doesn't necessarily need more than
two steps:
1. run r.watershed and output drainage direction and stream
segments. The key input parameter is "Minimum size of
exterior watershed basin". You probably also want to output
unique label (a catchment map) and flow accumulation (number
of cells that drain through each cell").
2. unless your point of interest happens to be at the
bottom of one of the catchments already mapped, run
r.water.outlet, with the drainage direction raster as
input. When doing this, view the stream segments layer - if
your point of interest is not exactly on a stream segment
you need to specify instead a nearby point that is on a
stream segment.
Hi
I can reaffirm Alister's suggestion. I use GRASS GIS all the time
for tasks like this with the r.watershed and r.water.outlet modules.
You might prefer to learn how to use GRASS independent of QGIS:
that removes one layer of possible problems.
And in any case, pay special attention to Alister's second point
above: your chosen drainage point must be exactly on one
of the stream lines from the first r.watershed stage. Zoom in very
close to the area of your known point, then click on the stream to
get the actual X-Y coordinates for the outlet point. And use those
coords in r.water.outlet.
The third issue that sometimes trips people up, (also mentioned
by Alister): the input to r.water.outlet is the flow direction
map from r.watershed
(not the DTM)
HTH
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--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
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