[GRASS-user] Building topologically correct stream network

Markus Metz markus.metz.giswork at googlemail.com
Thu Jul 23 02:15:41 EDT 2009

Hamish wrote:
> MMetz:
>>> What exactly is wrong with topology? The existence of these dangles?
> note the v.strahler help page makes special mention that if you used
> r.watershed you should check the topology. Maybe it makes a bitter deal
> out of that than is really the case?
> S:
>> Maybe there is nothing wrong.  On further reflection the area that
>> I am trying to use this procedure on is very flat and there are
>> intermittent streams and wetlands (site visit) all over the
>> study area.  These non-connected lines maybe showing
>> intermittentancy?
> do you see a "ladder effect" in the r.to.vect river network?
> (like a braided river with all joins at right angles)
> maybe that is the problematic topology the module is concerned about?
I can only think of these two possible problems - dangles and ladders. 
Dangles are caused by the method used to extract streams from a flow 
accumulation map whereas these ladders are an artefact of r.to.vect.

I'm regularly trying out different threshold values in r.watershed, only 
to get different streams to be vectorized, flow accumulation and 
drainage direction are not affected by the threshold option in 
r.watershed. I have an idea for a new module that would take flow 
accumulation and theshold as input and would then extract streams from 
it, both as raster and as vector (topologically clean in the v.strahler 
sense). This new module would work with both SFD and MFD flow 
accumulation and with r.terraflow flow accumulation. The problem is that 
the rough concept exists in my head only and I have no time right now to 
develop this module. All the code is there in r.watershed, it just needs 
to be reorganized a bit...

Markus M

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