[GRASSLIST:676] Re: Extracting vector river network from r.terraflow.

[Antonio]

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* martedì 08 luglio 2003, alle 16:28, Massimiliano Cannata scrive:
> Flow accumulation raster provide for every cell the number of cells that
> drain into it.
> If you have a value of 35478 it does mean that you have 35478 that give
> water to that cell: suppone you have a vector of four cell, every cell give
> his water to the next, so his flow accumulation will be 1 for the first
> cell, 2 for the second, 3 for the third and so on.
> 
> About processing your river network as a polyline, i  guess you have to
> select a treshold to extract your network from the accumulation (you keep
> only the cell with higth accumulation flow, hopefully the river!) than you
> have to trasform in vector.....
> 

Mmm, I considered this option. The sequence I used was:

echo '200 thru 563600 = 1' |r.reclass r.tf-7m-acc-mfd out=r.drainage.new
title="Drainage r.terraflow"

r.mapcalc r.drainage.idrog=r.drainage.new

r.thin input=r.drainage.idrog output=r.drain.idorg-thin

r.line .......

However r.thin nor r.line contemplate tuneing options so as to eliminate
supriuos flow accumulation areas that r.terraflow necessarly generates.
r.line complains that there are not thinned grouped cells in the raster
generated from r.thin. So it's not working.

The 200 thru 563600 interval is necessary so as to get a connected
network but it's producing networks where in some areas the rivers are
20-30 pixels wide. It would be interesting to have a feature that
could be instructed to pass in the middle of these wide areas or
something like that.

I also tried the r.ply option but it's not satisfing me as it just
creates vector lines that border the 30 pixel wide areas. I need a 1
pixel wide river network.

Any solution that someone can propose to my problem?

Thanks
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