[GRASS5] v.surf.rst - serious errors in DEM

[Maciek Sieczka]

From: "Jaro Hofierka" <hofierka at geomodel.sk>

> Hi Macej,
>
> Your data is pretty tough for any interpolation method, including RST.
> Points are extremely distributed and sharp fault means extreme change of
> elevation. The RST method has proved that it is an excelent method even in
> this situations, but limits do exist.

You are right. I tried all the interpolation methods available in Surfer,
with the same input data, and none (including min. curvature) produced an
acceptable result.

Minimum curvature performed well only when using "typical" elevation points 
(contour lines and hilltops) + fault lines. But when fault areas are 
represented by densely sampled elavation points, min. curv. fails same as 
any other method. Except triangulation
http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/surfer_triang_exag10.png
and natural neighbor
http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/surfer_natneigh_exag10.png
which gave excellent results, even with such a "tough data"!

> As I mentioned earlier I believe that the excelent result of Surfer in
> your example is not a result of interpolation per se, but data processing.
> I suggested a procedure that should help. Did you try it?

I'm afraid it is simply impossible to separately interpolate areas divided 
by faults and patch them subsequently, when input data looks like this 
http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/contours_faults.png 
(black - elvation data, red - fault lines). How do you think Jaro?

Summarising my amateur's remarks on DEM interpolation:
1. v.surf.rst could greatly benefit from supporting fault lines the way that 
Surfer or Surge does; then it could perform much better on areas with 
faults.
2. We are missing triangulation in Grass (wich was already rised by Helena I 
recall); I'm personally missing it badly - it showed to be perfect for 
heavily heterogenous data.

Maciek 


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