[GRASS-user] Local Relief Model tool

[Hamish]

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)

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