[GRASSLIST:599] Re: Understanding why r.flow gives me bad results.

[David Finlayson]

Your DEM may have little pits (closed basins) that stop drainage. 
Usually, these pits are artifacts of the DEM so you can use a program 
like r.fill.dir to "fill" the pits and created a "filled" dem.

Use the "filled" dem in r.flow and see if the results are better. r.flow 
is pretty picky about flat areas (see the notes in the manual).  You may 
have better luck with r.watershed and r.terraflow for extracting the 
river networks.

David Finlayson

Antonio G. - Geotronix wrote:

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> Hi.
> 
> I'm trying to compute a flowline path with r.flow so as to create a
> vector drainage pattern. However I'm not getting good results as r.flow
> is giving me a vector file in which the the flowline are very short and
> do not connect in the valleys. The valleys actually seem to have no
> flowlines while only very steep slopes do have flowlines. 
> My DEM does have flat areas (r.stats reports 500.000/2.800.000 zero 
> degree cells) so I presume it's this that stops the flowlines.
> 
> I must say though, that my DEM was built using only contour lines and no
> quoted points. I presume this is the cause of my 500.000/2.800.000 zero
> cells .....?
> 
> I built my DEM using r.surf.contour on a 7,5 meter square cell
> resolution for an area of 11,5 Km x 14 Km (total of 1800x1550 celled
> grid). Do you think my resolution is too low to get a valid flowline
> raster map? I read in the r.flow man page that centimeter resolution
> should be used. But if I use a centimeter resolution my PC would take
> 1000 light years to compute my DEM .... :-)
> I'm sure I missing something but I'm coming up to Gis technologies only
> recently so ...... not good at it yet.
> 
> Bye
> 
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-- 
David Finlayson
School of Oceanography
Box 357940
University of Washington
Seattle, WA  98195-7940
USA

Office: Marine Sciences Building, Room 112
Phone: (206) 616-9407
Web: http://students.washington.edu/dfinlays