Using the STk package for Kriging

[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.

Sure, I can write my own covariance function, but since I am a beginner
at Kriging I wonder why exponential and spherical are missing: is simply
lack of manpower or is there a reason why those functions are not
implemented?  Or maybe they are there under a different name or the like?

Specifically, I need a spherical variogram: if it is not there, are
there people here that have written one for themselves and would share
it?

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