[GRASS-user] Problems with r.surf.contour

[Markus Metz]

Dear Bianca,

On Thu, Mar 30, 2017 at 10:37 AM, Bianca Federici <bianca.federici at unige.it>
wrote:
>
> Dear Markus,
>
> thank you very much for your suggestion... I never used the option -a in
g.region command!

The -a flag for g.region ensures that the cell resolution is exactly as
given (5 meter in your case). That makes subsequent calculations easier
instead of having non-square cells. A side-effect is that the region might
be slightly enlarged in order to get the desired resolution which in turn
means that the vector is fully covered.

More important is that you set the region first, before you rasterize the
contour lines, and then don't change the region any more.
>
> The results improved a lot, but some problems are still present in the
linear interpolation between two contour levels.

r.surf.contour uses linear interpolation between contour lines. That means
there can be sudden changes in slope at the location of contour lines. You
can also try r.resamp.rst or r.resamp.bspline to use spline interpolation
which can give you smoother surfaces.

The contour lines seem to be at 25 meter steps, that is rather coarse. You
could augment the contour lines with a DEM, e.g. SRTM with a vertical
resolution of 1 meter in places with large gaps between contour lines, then
interpolate with r.resamp.rst or r.resamp.bspline.
>
> Here you may see the slope map and a slope profile that enhance the not
proper result:
>
> https://www.dropbox.com/s/7olimii9s0m21rh/slope_contourlevel.PNG?dl=0

That's the same image like in your first post?

Markus M

>
> I hope that you or somebody else have other suggestions ;-)
>
>     Bianca
>
>
> Il 27/03/2017 21:21, Markus Metz ha scritto:
>
> g.region is a very powerful tool with sometimes unexpected effects. See
suggested workflow below.
>
> On Mon, Mar 27, 2017 at 12:26 PM, Bianca Federici <
bianca.federici at unige.it> wrote:
> >
> > Dear all,
> > I'm writing to report a problem I noticed using r.surf.contour.
> > I created a DTM from contour lines, but I notice something a wrong
behaviour of the r.surf.contour resulting map.
> >
> > In detail I imported with v.in.ogr a shape file describing contour
lines every 25m of altitude
> > Then, I converted in raster (setting a resolution of the computational
region equal to 1m), and created the DTM ( setting a resolution of the
computational region equal to 5m) with r.surf.contour.
> > Either plotting a profile of the DTM and creating the slope map (with
r.slope.aspect), it's evident that r.surf.contour works bad.
> >
> > Here you can see the slope map, with the contour level in black, and
the position of the extracted cross section; you may notice artificial
steps mainly in corrispondence of the contour levels, instead of uniform
slopes:
> > https://www.dropbox.com/s/7olimii9s0m21rh/slope_contourlevel.PNG?dl=0
> > Here you can see the DTM profile along the cross section, that shows
the irregular morphology of the created DTM
> > https://www.dropbox.com/s/useoxstmceyulk6/DTM_cross_section.PNG?dl=0
> >
> > I tested it in GRASS 6.4.4, 7.0.3 and 7.2.0, working either in Windows
and in Linux.
> >
> > I hope that the developers team could correct this malfunction.
> > Best regards,
> >     Bianca
> >
> > PS: The used commands:
> > v.in.ogr ...
> > g.region -p res=1
> > v.to.rast input=contourlevel25m type=line output=contourlevel25m
use=attr column=ELEVATION
> > g.region -p res=5
> > r.surf.contour input=contourlevel25m output=dtm_contourlevel25m
> > r.slope.aspect elevation=dtm_contourlevel25m slope=slope_contourlevel25m
>
> Please try
> v.in.ogr ...
> g.region -p -a res=5 vect=contourlevel25m
> v.to.rast ...
>
> # do not change the region again
> r.surf.contour ...
>
> HTH,
>
> Markus M
>
>
> >
> > --
> > Eng. Bianca Federici, PhD
> > Assistant Professor in Geomatics
> >
> > DICCA - Department of Civil, Chemical and Environmental Engineering
> > Laboratory of Geodesy, Geomatics and GIS
> > University of Genoa
> > Via Montallegro, 1 - 16145 Genova ITALY
> > Phone: +39 010 353 2421 - Fax: +39 010 353 2546
> > E-mail:bianca.federici at unige.it  -www.dicca.unige.it/geomatica/ricerca
> >
> > Gter srl Innovation in Geomatics, Gnss and Gis
> > Spin-off of the University of Genoa
> > Piazza De Marini 3/61, 16123 Genova
> > Phone: +39 010 8694830 - Fax: +39 010 8694737 -
> > E-mail:info at gter.it  -www.gter.it
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> >
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