[GRASSLIST:1575] Re: interpolate nominal values

[Bernhard Sturm]

At 16:29 08.03.01 +0100, Edzer J. Pebesma wrote:

> > I want to interpolate nominal data (soil types as point information).
> > Which possibilities do I have, what's the best method? -Any
> > experiences, suggestions?
>
>They're hard to interpolate. Nearest neighbour (forming thiessen polygon) 
>seems
>a useful action; indicator simulation another one. Most interpolation methods
>are based on weighted averages, and are therefore only suitable to
>interval/ratio

but with the Thiessen Polygons you run into the trouble of producing 
'plateaus' and 'canyons' between observed points... (which are hardly to 
explain in the 'real' world) Thiessen seems to be a starting point of 
understanding spatial interpolation, but it's not the end of the story. 
According to my experience you can use other methods (Inverse Distance 
works good for surprisingly many applications), but you have to understand 
the mechanism of the interpolation model. Kriging is difficult to handle, 
due to the variograms one has to estimate before running an interpolation. 
For some application regularised splines with tension outperforms Kriging 
and IDW quite well... but it really depends on the situation. (I only have 
experience in precipitation interpolation, but not with soils...)

Bernhard
--
CDE, Centre for Development and Environment, University of Berne

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