[GRASS-user] Problem with slope modeling .

[Hamish]

> 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