[GRASS-user] Local Relief Model tool

[Eric Goddard]

On Thu, Sep 26, 2013 at 12:54 AM, Hamish <hamish_b at yahoo.com> wrote:

> Vaclav wrote:
>
> > Hi Eric,
> >
> > actually I was working on the same module (named r.local.relief), so
> > we have some duplication now. It is also based on [Hesse2010]. I was
> > about to commit it to GRASS Addons but I need to write documentation
> > first. For now, I'm adding the Python script into a attachment. > >
> > I've quickly tested both and they give slightly different results. The
> > other visible difference is the contours-to-really_smooth_elevation
> > step.
>
>
> Hi,
>
> could you discuss the advantage of the local relief method? I take it that
> the idea is to remove the background signal to highlight local detail?
> v.surf.rst or v.surf.bspline with a really big search window and loose
> tension might also be good for a smoothed backgroundto subtract away, and
> see also recent discussion about trying to get planar trend surfaces out of
> r.cog addon on the grass-dev ML (a work in progress).
> Should fine relief be removed as well? (so gate filter and not just a low
> or high pass one)
>
>
> It seems to me that the contouring step is needlessly lossy and artifact
> prone, and that you might get better results using the r.surf.contour
> module to recreate a raster from the contour lines. Or perhaps better use a
> {v,r}.random sampling technique as input to one of the v.surf spline
> modules or r.surf.nnbathy -- less artifacts than interpolating from contour
> lines. Also note that the contour method will flatten off the tops of
> features which are smaller than your contour step level. (so choice of
> contour step size becomes very important)


The thought behind the contouring step is that when you apply a low pass
filter and subtract it from the DEM, the large scale features (based on the
selected kernel size) are eliminated and the small-scale features are
smoothed, resulting in bias toward small features and an underestimation of
the magnitude of the local relief elevations as the spatial extent of the
feature increases. By getting the 0-meter contours, converting them to
points, and getting the elevations of the original DEM for those points,
you're basically able to cut the large-scale features out of the original
DEM, leaving the local relief behind without smoothing.

>


> See also http://grasswiki.osgeo.org/wiki/Contour_lines_to_DEM
> for an analysis and comparison of interpolation methods for creating
> surfaces from contour lines. (spoiler: the designed for the task
> r.surf.contour wins)
>
>
> regards,
> Hamish
>
>
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