[GRASSLIST:1578] Re: interpolate nominal values

[Bernhard Sturm]

It's getting OT, but I think it's worth the discussion:

>   Hold on folks! Before we get too far down this side path, pay attention,
>please, to the data type: Nominal.
>
you are right. I was completely absorbed by my precipitation data.... but 
then again you could in fact use the thiessen polygons to interpolate 
nominal data, as there are no weights on the interpolated values. you just 
draw boundaries between type1 to type2... (the border line between the two 
types is only a function of the distance between type1 and type2 sample 
point, and will not be weighted by any calculated mean value, hence it must 
be possible to draw such a map...)
And if you take it one step further. There is one way you could treat your 
nominal data as a ratio data type: you assign to soil type 1 the number 1; 
soil type 2 is 2, and so on. Then you treat these numbers as ratios and 
interpolate them to a surface (IDW for instance), now you will get a 
surface with a lot of fractions between 1...2, if you re-class your surface 
in such a way that you say that everything <1.5 is assigned to soil type 1 
and everything >=1.5 is soil type 2 then you could extract your isolines, 
showing you the boundary of the two soil types... I am aware that this is 
by no means a proper application, but I could imagine that one would then 
be able to draw a soil type map (it all depends on the quantity of your 
sample points, if you have observed enough points you must end up with 
quite a reliable map...).
Does anybody know if this method is completely wrong, or could it be used 
to some extent?
I believe I will have to re-read the first chapters of "Applied 
Geostatistics"...
*smirk*

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

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