
From:  Julien Bect 
Subject:  Re: Using the STk package for Kriging 
Date:  Fri, 11 Nov 2016 14:02:30 +0100 
Useragent:  Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Icedove/45.4.0 
Le 11/11/2016 à 12:33, Francesco Potortì a écrit :
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?
Hi Francesco,You're right, the number of available covariance functions in STK is currently quite limited.
The main reason is that the way covariance functions are written in STK makes it difficult to expand the catalog of available covariance functions properly without duplicating a lot of code (code that handles a computation cache). Future evolutions of STK will make it much easier. Which is why I was wiaiting for this evolution to happen before adding more covariance functions... The main reason why this hasn't happened yet is lack of manpower, as you said. Recent developments in STK were simply heading in a different direction ;)
Now, to answer your request more specifically :The "exponential variogram" is actually the same thing as the Matern 1/2 covariance function, so it is available as a special case of the Matern covariance function (if you're interested in parameter estimation, however, you will have to make a wrapper that "fixes" the shape parameter to 1/2).
The spherical covariance is not available yet, but I will add it soon, since you need it ;)
@++ Julien
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