[GRASS-user] r.terraflow -> streams ?
dylan.beaudette at gmail.com
Wed May 17 13:54:40 EDT 2006
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.
Good points Kessie.
DEM creation, filtering, and all subsequent analysis can be somewhat of an art
some interesting articles surface when searching for the reference that you
> 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 . 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.
> >  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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> Ksenia E. Konwicki, RPF
> 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
Soils and Biogeochemistry Graduate Group
University of California at Davis
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