[GRASS-user] v.net vector orientation when creating a stream network with gauges

[Bartolomei.Chris]

Good afternoon,
I was hoping someone could provide some insight as to how v.net works.  I have a set of (438) watershed boundaries and their DEMs from which I have created the stream vectors from the DEM rasters (r.watershed, r.thin, r.to.vect, v.clean, etc.)  I also have the set of USGS stream gauges as points from which I extract the gauges within each watershed as a separate map. I am able to use v.net op=connect to create a network with the stream vectors and watershed gauge points no problem and v.path successfully creates paths from one gauge to the next IF the forward and backward costs are the same (cost is length).
What I am trying to do is measure the distance between a pair of gauges that are in the same flow path. I am automating this so there is no user input as to which pair to select, therefore I would like to use v.net.allpairs and then analyze the resulting table and select only those which have a route.
If I set the fwd cost to length and the backwards cost to -1 to shut off going upstream after traversing downstream, then the paths fail – which in many cases is good, but the paths between gauges that are clearly up/downstream from each other (along the same flow path) fail as well. If I reverse the forward and backward cost columns the path calculations still fail.
My thoughts are that the stream vectors do not have the correct forward and/or backward orientation (they are probably mixed) in the network. Is there a way to see or set the orientation of the stream vectors? Does anyone have experience with this?
When I created the stream vectors, I didn’t include the 3D data, if it were included in the stream vector would the forward/backward orientation then go from higher elevation to lower?

I’ve looked at (https://grasswiki.osgeo.org/wiki/Vector_extract_upstream_network) and I seem to lose a lot of (stream) lines when the network is created this way which I feel may be related to the vector orientations as well.

Any assistance would be greatly appreciated!

Chris



Chris Bartolomei P.E.
Engineer/Scientist
ENSCO, Inc.
4849 North Wickham Road
Melbourne, FL 32940 (official office)
5038 Park Rim Drive
San Diego, CA 92117 (home office)
(858) 581-3005
bartolomei.chris at ensco.com

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