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Hi Thomas:<br>
<br>
<div class="moz-cite-prefix">On 02/13/2015 10:28 PM, Thomas Adams
wrote:<br>
</div>
<blockquote
cite="mid:CAGxgkWi3UvL9dbxvP05J+sq9vK3C9zUc0CxFenQ2YwnYYDQKZQ@mail.gmail.com"
type="cite">
<div dir="ltr">Micha,
<div><br>
</div>
<div>No, after looking at what's going-on in more detail, I
think the DEM is too coarse (even at 90m), so the flow
direction and accumulation is mis-directed in one critical
area of the watershed. I tried using r.carve, but it is taking
forever — after 15 minutes, no advancement of the progress
bar…</div>
<div><br>
</div>
</div>
</blockquote>
<br>
I don't see why carving into a DEM should cause r.watershed to run
more slowly, unless you have carved out only a section of some of
the streams. If your carving does not continue right to the outlet
of the stream (i.e. to an ocean, or to the edge of the region) then
r.watershed would actually have to fill in that carved out stream to
find a flow path. That could cause the performance hit. <br>
<br>
Additionally, are you using GRASS 7.0? As you probably know some
substantial improvements to the algorithms were introduced in 7 for
several modules, r.watershed among them.<br>
<br>
And third, why use the D8 flow direction when MFD is available
(again in GRASS 7.0)? That could also be causing what you refer to
as breaks in the channels. MFD is especailly good, I believe, in
flat areas.<br>
<br>
In any case, Keep up posted on your progress.<br>
Thanks,<br>
Micha<br>
<br>
<blockquote
cite="mid:CAGxgkWi3UvL9dbxvP05J+sq9vK3C9zUc0CxFenQ2YwnYYDQKZQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>I did not want to have to do this for my testing, but I'll
probably try at 3 or 10 meter — lots of pixels for my basin!
The problem, in general, for me is that I want to apply my
techniques at international locations where I probably won't
have the benefit of higher resolution DEMs, so I need to
develop something a bit more robust…</div>
<div><br>
</div>
<div>Cheers!</div>
<div>Tom</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Fri, Feb 13, 2015 at 1:15 PM,
Micha Silver <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:micha@arava.co.il" target="_blank">micha@arava.co.il</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="direction:ltr" bgcolor="#FFFFFF"
text="#000000"><span class=""> <br>
<div>On 02/13/2015 08:48 PM, Thomas Adams wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Stefan,
<div><br>
</div>
<div>A fair question; since I know the stream
topology from personal experience it is clear
that there should be no break in the stream
network and the flow accumulation grid should
reflect that. I am seeing the flow accumulation
values break at a point where they should
continue to accumulate downstream.</div>
<div><br>
</div>
</div>
</blockquote>
<br>
</span> Are these breaks possibly caused by null pixels
in the DEM? <br>
<br>
<blockquote type="cite">
<div>
<div class="h5">
<div dir="ltr">
<div>Tom</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Fri, Feb 13, 2015
at 11:43 AM, Stefan Lüdtke <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:sluedtke@gfz-potsdam.de"
target="_blank">sluedtke@gfz-potsdam.de</a>></span>
wrote:<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px
#ccc solid;padding-left:1ex">-----BEGIN
PGP SIGNED MESSAGE-----<br>
Hash: SHA1<br>
<br>
Hi Tom,<br>
<br>
just out of curiosity, what do you mean by
"break in the flow<br>
accumulation"?<br>
<br>
Cheers,<br>
<br>
Stefan<br>
<div>
<div><br>
On 02/13/2015 07:12 PM, Thomas Adams
wrote:<br>
> Hello all!<br>
><br>
> I'm making use of the flow
accumulation grid in GRASS 6.4.5<br>
> generated from r.watershed using
the SFD (D8) flow algorithm. The<br>
> DEM has a 250m spatial
resolution. What I'm getting is a
break in<br>
> the flow accumulation in a few
locations which is causing me<br>
> serious problems with subsequent
processing (with help from some<br>
> here, I have put together some
scripting to generate a pixel<br>
> connectivity file for a
distributed hydrologic model).<br>
><br>
> Besides going to a higher
resolution DEM, are there any thoughts
as<br>
> to how I can eliminate these flow
accumulation breaks?<br>
><br>
> Thank you, Tom<br>
><br>
> --<br>
><br>
><br>
><br>
</div>
</div>
>
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