[GRASS-user] Problem with slope modeling .

[Dylan Beaudette]

On Monday 30 April 2007 06:01, Radomir wrote:
> Dnia 26-03-2007, pon o godzinie 20:30 +1200, Hamish napisał(a):
> > > Radomir wrote:
> > > > I'm preparing the slope and elevation map from digitised isolines.
> > > > I'm using v.surf.rst to do that with the following options:
> > > >
> > > > v.surf.rst input="izolinie_punkty" layer=1 zcolumn="wysokosc_m_npm"
> > > > dmax=3.174812 dmin=0.634962 elev="mapa_wystaw" slope="mapa_spadk"
> > > > maskmap="maska" zmult=1.0 tension=40. segmax=40 npmin=300
> > > >
> > > > The screenshot from small part of resulting map is here:
> > > > http://mort.no-ip.org/screen.png
> > > >
> > > >
> > > > On this screen you can see the valley in the center. I expect that
> > > > the slope between two isolines on the bottom of the valley will be
> > > > similar. But it is changing and gives an effect of "wave" between
> > > > isolines.
> >
> > Mars wrote:
> > > I presume the Isolines are representative of Elevation.
> >
> > and the displayed raster, legend, and profile show the slope.
> > it might help to show a profile using the elevation map. (it is hard to
> > tell if the slope stays monotonically increasing in your profile as
> > slope will always be positive)
> >
> >
> >
> > profiler notes:
> > - y-scale is ranged to max/min of map, not max/min of profile?
> >   ($elevrange from r.univar instead of r.profile in gis.m/profile.tcl)
> >   Not sure which is best. Both ways have some merit.
> >
> > - in your screenshot the axes tick labels are not formatted cleanly.
> >   I've just prettified those in CVS, please test.
> >
> > - BUG: if profile ends outside of the raster map off in NULL-land,
> >   the totaldistance calculation is calculated to the end of the arrows,
> >   but the end of real data in the plot is stretched to the far right end
> >   regardless. e.g.: Zoom out so your raster is a postage stamp sized box
> >   in the middle, then make some profiles from map canvas corner to
> >   corner. The start of the data is positioned ok. Also it should break
> >   the plot at NULL data, not draw a line beween the last known good data
> >   either side of the NULLs.
> >
> > - it would be possible to add a title like "Profile for $mapname" at the
> >   top of the profile. Is there demand for this? Better to have it in the
> >   window title than on the plot itself?
> >
> > Maciek wrote:
> > > I had this problem too when interpolating DEM from contour lines with
> > > RST. Try to interploate DEM with r.surf.nnbathy from GRASS WIKI
> > > add-ons page and let us know if the artifact hollow between 2 adjacent
> > > contour lines still remains. I'm not 100% sure it will help. It might.
> >
> > v.surf.rst is designed for semi-evenly distributed series of data points
> > (at least locally, it is fine for data density to smoothly change), not
> > contour lines.
> >
> > running v.to.points + v.surf.rst tends to bias the surface to the
> > isolines. Output the treefile= segmentation map and curvature maps to
> > see the effect in detail. Also in the GRASS book, if you have access to
> > it.
> >
> > as Maciek wrote, you should get better results with r.surf.nnbathy, or
> > you can try r.surf.contour (see hints about that posted in the last day
> > or two to this list for r.surf.contour).
> >
> > You might try r.contour with your output raster map at levels half way
> > between your original isolines to get a nice visual check of how the
> > interpolation went. (create new contour lines 1/2 way between the
> > original ones and overlay them)
> >
> >
> >
> > Hamish
>
> Maciek, Hamish, Mars,
> Thank you for your comments.
>
> I've interpolated isolines with use of r.surf.nnbathy:
>
> r.surf.nnbathy alg=nn input=izolinie output=nn_elevation
>
> unfortunately, the results are even worse that those which comes from
> RST method. After "translating" it into the slope map (r.slope.aspect) I
> have the following result:
>
> http://mort.no-ip.org/grass/indexgrass.html
>
> As You can see, I still have "waves" between isolines. On the graph you
> will find those "waves" really large.
>
> The r.surf.contour methods gives much worse results then RST (see the
> Elevation graph).
>
> I've run r.contour with my RST output map as Hamish suggested ( see the
> last image on the http://mort.no-ip.org/grass/indexgrass.html )
>
> It was really interesting to analyse the output vector map. It seems
> that when the distance between two isolines is "large" GRASS "puts the
> next isoline" very close to the previous and this is how the "waves"
> comes from.
>
> What sholud I do next?? Please for comments and suggestions.
> Best Regards,

I do not know what is causing this behaviour, but there is a well-documented 
problem with USGS dem data that causes similar artifacts:

(see the image at the bottom of the page)
http://169.237.35.250/~dylan/gdalwarp/image_matrix.html

dylan

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