[GRASSLIST:654] Re: Extracting vector river network from r.terraflow.

[Massimiliano Cannata]

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.....

"Antonio G. - Geotronix" wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> HI.
>
> After trying r.flow and r.watershed to compute river networks over my
> DEM I've finally tried r.terraflow with the 28/06 snapshot.
> r.terraflow is finally giving a better stream network than the former
> and this is visible by the flow accumulation raster layers.
> However, what I need is a vector river network layer. I presume I have
> to use the flow accumulation layer provided by r.terraflow as input but
> I don't know how to process it. Must I use r.thin + r.line. But how? I
> gave it a try but got errors from r.line:
>
> GRASS:~ > r.line input=r.tf-7m-thin-accu output=v.tf-7m-thin-accu
> Opening files
> Performing extraction
> update_list:  crowded cell 8H (89,321)
> ERROR: cell file is not thinned properly.
>        Please run r.thin
>
> Maybe I have to reclass the categories?
>
> BTW. The flow accumulation categories should express the amount of
> water running through a cell? I saw the categories are rather
> large 0 --> 631571. But what are they exacly expressing. If I get
> a value of 631000 for a cell of a accumulation layer outputted by
> r.terraflow, how must I interpret it?
>
> Thanks
> Bye
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.2.2 (GNU/Linux)
>
> iD8DBQE/Cs+FIRaQQEznKxURApevAJ4tYMHm3xpCHXAQuItvUq3wmSrFtgCeODq1
> 1485LhmRWLDLA4/8+mkWkDg=
> =5HQQ
> -----END PGP SIGNATURE-----

--
-------------
Ing.  Massimiliano Cannata
Istituto di Scienze della Terra - SUPSI
C.P. 72 - CH-6952 Canobbio (Ticino, Switzerland)
Tel +41 91 /935 12 25 - Fax +41 91 /935 12 09
eMail: massimiliano.cannata at ist.supsi.ch
Internet: http://www.ist.supsi.ch