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<div class="moz-cite-prefix">On 10/20/18 12:55 AM, Rich Shepard
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:alpine.LNX.2.20.1810191449320.27422@salmo.appl-ecosys.com">On
Fri, 19 Oct 2018, Rich Shepard wrote:
<br>
<br>
<blockquote type="cite">Gotcha'. I'll do this instead of using
r.flow. (Running r.watershed rather
<br>
than r.flow.)
<br>
</blockquote>
<br>
Well, this is not working for me (see attached .png). Most of
the upper
<br>
left area is quite flat, especially east of the river.
<br>
<br>
The manual's explanation of negative numbers, that they possibly
have
<br>
surface runoff from outside the area, doesn't seem likely here
because the
<br>
drainage basin's watershed divide is the outline of the area.
<br>
</blockquote>
<p><br>
</p>
<p>I have always found that the best way to do watershed
delineation, to get streams and flow accumulation is to set the
computational region a *bit larger* than the basin you are
examining, and use a DEM that covers the full region. i.e. don't
second guess the module. Then all pixels within the basin will
have positive flow accum values. (Only pixels outside the main
basin, which are in partially covered, adjacent basins will have
negative values).</p>
<p>The final step could be to mask out all areas outside the basin
of interest. <br>
</p>
<p><br>
</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:alpine.LNX.2.20.1810191449320.27422@salmo.appl-ecosys.com">
<br>
I used the -b option in the command (and the -m option on this
host) and
<br>
ask for ideas on how to get the legend to expand the very narrow
non-black
<br>
band to show the colors on the map, and ignore all the negative
values.
<br>
<br>
Also, if there are additional input maps I need to have
calculated I want
<br>
to learn which ones and how best to prepare them.
<br>
<br>
</blockquote>
<p><br>
</p>
<p>Depending on what results you need, you might apply a
"depression" raster if there are true depressions in you region
(like the Dead Sea in my case), but I don't think you have
anything like that. And of course the threshold parameter is very
important - determines how dense the final stream network will be.</p>
<p> <br>
</p>
<blockquote type="cite"
cite="mid:alpine.LNX.2.20.1810191449320.27422@salmo.appl-ecosys.com">Rich
<br>
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<br>
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</blockquote>
<pre class="moz-signature" cols="72">--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918</pre>
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