[R-sig-Geo] Parameters in fitting variogram automatically in gstat

[Edzer Pebesma]

Dear Kai,

you might want to look at function autofitVariogram in R package
automap, which exactly tries to do this. It uses reasonable default
(starting) values, but as any algorithm, it may not work well with
particular input data that is extreme in some sense.
--
Edzer

On 05/10/2010 06:52 AM, Kai Zhang wrote:
> Hi folks,
> 
> I am working on a problem dealing with fitting variogram automatically due to a large dateset. Gstat R package is used to conduct this work. Part of this work is to compare performance of different kriging methods. The comparison criteria is RMSE. I am using fit.variogram function for ordinary kriging, and fit.lmc for co-kriging. I have to specify a theoretical variogram at first. The problem is that results are sensitive to initial values specified in theoretical variograms, mainly "Range". Gstat manual points out "If fitting the range(s) is part of the job of this function, the results may well depend on the starting values, given in argument model." (for fit.variogram function). My question is how to choose starting values. I did take a few data to check variograms, and found they had different "range" values. I am not sure what starting values I should choose, say a small "range" value or a large "range" value. 
> 
> Best regards,
> Kai
> 
> 
> 
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de