[R-sig-Geo] Parameters in fitting variogram automatically in gstat
nitmithv at yahoo.com.cn
Mon May 10 06:52:01 CEST 2010
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.
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