<html style="direction: ltr;">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<style id="bidiui-paragraph-margins" type="text/css">body p { margin-bottom: 0cm; margin-top: 0pt; } </style>
</head>
<body bidimailui-charset-is-forced="true" style="direction: ltr;">
<p><br>
</p>
<div class="moz-cite-prefix">On 09/11/2022 8:53, Alister Hood via
Qgis-user wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAF6hL+Ugy37wLeESEinoVizSxcDhNh+7hmoTDOOiWDWXZJCVhQ@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">Date: Wed, 9 Nov 2022
11:04:02 +1300<br>
From: "Tony Shepherd \(FarmMaps NZ\)" <<a
href="mailto:tony@farmmaps.nz" target="_blank"
moz-do-not-send="true" class="moz-txt-link-freetext">tony@farmmaps.nz</a>><br>
To: "'Qgis Users List'" <<a
href="mailto:qgis-user@lists.osgeo.org" target="_blank"
moz-do-not-send="true" class="moz-txt-link-freetext">qgis-user@lists.osgeo.org</a>><br>
Subject: [Qgis-user] Watershed - Catchment area upstream of
a point<br>
Message-ID: <003101d8f3be$04ed8380$0ec88a80$@<a
href="http://farmmaps.nz" rel="noreferrer" target="_blank"
moz-do-not-send="true">farmmaps.nz</a>><br>
Content-Type: text/plain; charset="us-ascii"<br>
<br>
Hi All<br>
<br>
<br>
<br>
I'm wanting to use a DEM/DTM to automatically map the
watershed/catchment<br>
area upstream of a known point.<br>
<br>
<br>
<br>
</blockquote>
<div>Yes, Grass is preferable because the integration with
QGIS works reliably. It doesn't necessarily need more than
two steps:</div>
<div><br>
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").</div>
<div>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.</div>
<div><br>
</div>
</div>
</div>
</blockquote>
<p><br>
</p>
<p>Hi</p>
<p>I can reaffirm Alister's suggestion. I use GRASS GIS all the time
for tasks like this with the <font face="Courier New, Courier,
monospace">r.watershed</font> and <font face="Courier New,
Courier, monospace">r.water.outlet</font> modules.</p>
<p><br>
</p>
<p>You might prefer to learn how to use GRASS independent of QGIS:
that removes one layer of possible problems.</p>
<p><br>
</p>
<p>And in any case, pay special attention to Alister's second point
above: your chosen drainage point <b>must</b> be exactly on one
of the stream lines from the first <font face="Courier New,
Courier, monospace">r.watershed</font> 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 <font face="Courier New, Courier, monospace">r.water.outlet.</font></p>
<p>The third issue that sometimes trips people up, (also mentioned
by Alister): the input to <font face="Courier New, Courier,
monospace">r.water.outlet</font> is the <b>flow direction</b>
map from <font face="Courier New, Courier, monospace">r.watershed</font>
(not the DTM)</p>
<p><br>
</p>
<p>HTH</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:CAF6hL+Ugy37wLeESEinoVizSxcDhNh+7hmoTDOOiWDWXZJCVhQ@mail.gmail.com">
<div dir="ltr">
<div class="gmail_quote">
<div>Regards,</div>
<div>Alister</div>
</div>
</div>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
Qgis-user mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Qgis-user@lists.osgeo.org">Qgis-user@lists.osgeo.org</a>
List info: <a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/qgis-user">https://lists.osgeo.org/mailman/listinfo/qgis-user</a>
Unsubscribe: <a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/qgis-user">https://lists.osgeo.org/mailman/listinfo/qgis-user</a>
</pre>
</blockquote>
<pre class="moz-signature" cols="72">--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918</pre>
</body>
</html>