[GRASSLIST:1576] Re: interpolate nominal values

[Rich Shepard]

On Thu, 8 Mar 2001, Bernhard Sturm wrote:

> 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.

  Hold on folks! Before we get too far down this side path, pay attention,
please, to the data type: Nominal.

  I know that you all know this, but I'll provide a quick review from basic
statistics on data types.

	Nominal:  Qualitative rather than quantitative values, and therefore
incapable of any arithmetic operations (including interpolation to a 3D
surface. Names of soil types cannot be manipulated as numbers.

	Ordinal:  Quantifies by ordering on a linear scale, but not by
magnitude. An example might be soil color, ranging from the blue-grey gleyed
hydric soils through yellows, reds and browns to black, organic soils. Each
color can be assigned a number along that scale, but the numbers can be used
only to determine which ones are greater than others.

	Interval:  Quantifies by defining relative position on an interval
scale, but without reference to a fixed point. Temperature measurements are
a good example. 20 C is twice the number of 10 C, but that doesn't mean it's
twice as hot.

	Ratio:  Measurements on a scale with a true zero and equal
intervals. Precipitation is a good example. There can be 0 cm of rain, and
10 cm of rain is twice the amount of 5 cm of rain.

  The only way you can interpolate nominal data to a surface is if your
units of measurement is furlongs per fortnight. :-)

Rich

Dr. Richard B. Shepard, President

                       Applied Ecosystem Services, Inc. (TM)
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