Re: Using the STk package for Kriging

[Francesco Potortì]

Francesco Potortì:
>>> I am experimenting with Kriging interpolation and I am studying the STK
>>> package.
>>>
>>> I am surprised at the fact that the provided covariance functions are
>>> Metérn, Gauss, and empirical.  The commonly used exponential and
>>> spherical models are missing, like other common models.

Julien Bect:
>I have added the exponential and spherical covariance functions to STK 
>
>It will be available in STK 2.4.0, which I hope should be released 
>soon... (within a month or two).
>
>In the meantime, you can try the beta version that is available from 
>SourceForge: <http://people.ku.edu/~gbohling/cpe940>.

Thanks a lot, my colleagues are working on it now!

Since I am at it, here are some comments on the STK package.

I have had some difficulty in using STK because the concepts and
terminology are different form what I have read in the literature.  The
main example is the introductory paper on Kriging by Geoff Bohling that
you find at <http://people.ku.edu/~gbohling/cpe940>.  But I have found
the same concepts and terminology used in other papers and in the
"Kriging" and "variogram" Wikipedia pages.

Specifically, your code does not mention the variogram and does not
directly use parameters such as nugget, sill and range.  Since I am a
beginner at this, I would appreciate if you could please help me check
that I have correctly understood how to use the STK code.

For example, when using the new spherical variogram model, these are the
parameters that I should use:

sill is the long-range variance
nugget is the variance at null distance
range is where the variance becomes practically equal to the sill

model.lognoisevariance = log(sill);
model.name = 'stk_spherical_iso';
model.param = [log(sill-nugget); -log(range)];

Is that correct?

I think that the learing curve for the STK package would be much
smoother if the concepts of variogram, sill, nugget and range were
explicitely mentioned and an example were given on how to convert them
for use in the various models in STK.

Thanks again.

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