[GRASS-user] r.terraflow -> streams ?

[Dylan Beaudette]

On Wednesday 17 May 2006 10:22, Ksenia Konwicki wrote:
> Just a quick comment regarding DEM accuracy.
>
> It is important to understand how a DEM was generated when considering
> accuracy. Was it derived from points or contours, what interpolation
> process was used, is the data prone to striping artifacts?
>
> As an example, here in BC, point and breakline data - both hard and soft
> breaklines - are provided as raw elevation data. We have done a fair number
> of trials regarding how to create the most representative raster DEM. We
> have looked at IDW, RST with various tension settings, and kriging using
> point data, as well as deriving point data from the breaklines and creating
> a TIN with breaklines interpolated to raster. The nature of the raw data
> makes the TIN the optimal method - in essence the data was collected for
> that DEM creation method. Also the nature of the data collection leads to
> striping artifacts (see Albani & Klinkenberg, 2003) which must be removed
> before the DEM is considered hydrologically sound and representative. This
> is one factor that I have learned over the past year, without a high
> quality DEM hydrologic modelling is compromised, no matter what flow
> direction/flow accumulation algorithm you use.
>
> Kessie.
>

Good points Kessie.

DEM creation, filtering, and all subsequent analysis can be somewhat of an art 
form.

some interesting articles surface when searching for the reference that you 
mentioned:

http://scholar.google.com/scholar?hl=en&lr=&client=firefox&q=Albani++Klinkenberg%2C+2003&btnG=Search

Cheers,

Dylan



> Dylan Beaudette wrote:
> > On Wednesday 17 May 2006 00:42, orkun wrote:
> > > Dylan Beaudette wrote:
> > > >On Tuesday 16 May 2006 02:59, orkun wrote:
> > > >>hello
> > > >>
> > > >>can I  produce stream map using r.terraflow maps ?
> > > >>
> > > >>regards
> > > >
> > > >here is one such example:
> > > >
> > > >http://casoilresource.lawr.ucdavis.edu/drupal/node/166
> > > >
> > > >cheers,
> > >
> > > thank you
> > >
> > > I looked the site you directed me.
> > > Compared to r.watershed, r.terraflow produced stream map much faster.
> > > But there  seems to be broken streams. Although I lowered the
> > > accumulation threshold value, broken streams still exist.
> > > How can I get unbroken streams using r.terraflow.
> >
> > Hi,
> >
> > You could try computing the line lengths, and throwing out features that
> > are less that a given length:
> >
> > v.to.db ...
> > v.extract ...
> >
> > Or.. it would be even better to characterize stream networks according to
> > their "order" , based on the number of contributaries feed into the main
> > channel. Then, keep all streams of order _n_ or greater.
> >
> > > Since I worked on DEM quality, I saw some interesting features
> > > in the other part of the site. Does "DEM vs. Field Slope" show
> > > DEM's quality.
> >
> > I think that you might be referring to this image:
> > http://casoilresource.lawr.ucdavis.edu/drupal/node/189?size=_original
> >
> > The title is a little confusing, as it is a comparison between slope
> > values measured in the field with a clinometer compared with slope values
> > computed from a DEM via r.slope.aspect . The take-home message from that
> > graph was that the DEM-computed slope values tended to under estimate
> > field measured values [1]. Also that a comparison between DEM and field
> > slope seemed to work best when comparing average slope within a 20 meter
> > radius as compared to using the DEM derived slope value at the exact
> > location of the field measurement.
> >
> > [1] note that field measurement of slope is not an exact science, and is
> > certainly not conducted at the same scale at which the DEM for this area
> > was created (10 meter grid) .
> >
> > Cheers,
> >
> > --
> > Dylan Beaudette
> > Soils and Biogeochemistry Graduate Group
> > University of California at Davis
> > 530.754.7341
> >
> > _______________________________________________
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> > grassuser at grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
>
> --
> Ksenia E. Konwicki, RPF
> Ecologist
>
> Timberline Forest Inventory Consultants
> 1579 9th Avenue
> Prince George, BC   V2L 3R8
> P (250)562-2628
> F (250)562-6942
> E kes at timberline.ca

-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341