[GRASSLIST:1579] Re: interpolate nominal values

[Rich Shepard]

On Thu, 8 Mar 2001, Bernhard Sturm wrote:

> 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...)

Bernhard,

  While this might be computationally feasible, I suggest that it is
meaningless in the real world.

  Soil maps (and soil taxonomy in general) are best represented either by
vector polygons (the way they're drawn on paper maps) or raster regions that
more accurately reflect the transition zone between one soil type and the
adjacent one.

  Now, if you had ratio data representing, for example, samples of plant
species' densities and height (just to pick some arbitrary parameters for
the sake of this discussion) you could interpolate those data and drape the
soils layer over them to see if there were meaningful patters in the
relationship.

  Perhaps my point will be better understood if I present an absurd example.
(Don't try this at home! It can be done only by experts on a closed course.)
Suppose you had point (sites) data of fast-food restaurants (i.e., junk food
vendors) from a typical American medium size city. The nominal data
categories include "McDonalds", "Burger King", "Taco John", etc. Now,
regardless of whether you use IDW, Veronoi diagrams/Thiessen polygons, or
Krieging, you interpolate a 3D surface of these points. How do you interpret
the results? What will it tell you (or me, for that matter)?

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
              Making environmentally-responsible mining happen. (SM)
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