Tom,
Hopefully the higher spatial resolution data will also have higher vertical
resolution.
That said, if you have any idea where the stream channels are, for example a
vector file, in the flat areas you can "burn" them into the raster using
r.mapcalc. So you can rasterize vector streams (v.to.rast) and then thin the
raster streams (r.thin) and use r.mapcalc to subtract a constant value where
the raster stream map is. If you do not thin the raster you will get an error,
as there will be ambiguous flow paths (endless loops).
QGIS has a nice feature in its vector to raster tool. It will do the burn for
you directly from the vector file into your DEM. You should make a backup of
your DEM though first, since it edits the original file. You just add a field
to the vector streams for a desired elevation. Then you select that elevation
field in the vector to raster dialog and select your DEM as the output raster.
After saying that yes it is OK to overwrite your DEM, it will burn the vector
elevation. I have only used this to edit small areas on a raster, so I can not
tell you if the burnt channel is sufficiently thinned or not. One of the big
problems with using flow algorithms is that bridges and roads will frequently
act like dams, causing the flow to be rerouted outside of the actual channel.
To prevent this you can digitize a vector map of places where you want to cut
through these "dams" and set a field with desired elevations as described above.
-Thayer
From: Thomas Adams <[email protected]>
To: Thayer Young <[email protected]>
Cc: "[email protected]" <[email protected]>
Sent: Friday, February 13, 2015 5:48 PM
Subject: Re: grass-user Digest, Vol 106, Issue 34
Thayer,
Thank you for the suggestion; I just tried it with the same result. I think the
area just happens to be a difficult area, that is pretty flat…
Cheers!Tom
On Fri, Feb 13, 2015 at 1:54 PM, Thayer Young <[email protected]> wrote:
I don't know if you have tried this yet, but you may also want to look at
r.terraflow. It does both D8 (single flow direction) and multiple flow
direction (flow is partitioned, according to the steepness of slope, to all
directions that are lower than the central cell). Supposedly you can also
switch from MFD to SFD once flow exceeds a threshold, but I have not been
successful at doing this. It would be a simple simulation of channelization
though.
Just set the D8 flag in the options tab, otherwise it will give you MFD by
default.
-Thayer
Date: Fri, 13 Feb 2015 11:12:30 -0700
From: Thomas Adams <[email protected]>
To: "[email protected]" <[email protected]>
Subject: [GRASS-user] Question about r.watershed and flow accumulation
grid
Message-ID:
<cagxgkwgefettcjzwki3jp8wj79eb0j3fzsf6hgzt8m9-dgc...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hello all!
I'm making use of the flow accumulation grid in GRASS 6.4.5 generated from
r.watershed using the SFD (D8) flow algorithm. The DEM has a 250m spatial
resolution. What I'm getting is a break in the flow accumulation in a few
locations which is causing me serious problems with subsequent processing
(with help from some here, I have put together some scripting to generate a
pixel connectivity file for a distributed hydrologic model).
Besides going to a higher resolution DEM, are there any thoughts as to how
I can eliminate these flow accumulation breaks?
Thank you,
Tom
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