[R-sig-Geo] nugget in variogram

[Edzer Pebesma]

On 09/30/2010 04:13 PM, Arantzazu Blanco Bernardeau wrote:
> Hello everybody
> I am new on geostatistics so maybe my question is trivial or is
> already answered (I can not find the solution in the thread list). I
> have a variogram where I did not fix the nugget, because doing it
> was giving "singular models". I would like to know the nugget ratio
> of my variograms but I don't know how to get then the nugget value.
> So I did look in the output of variogram and did take the minimum
> value of gamma. Is this correct?

Well, it's not the usual way, where you would fit a model:

library(gstat)
data(meuse)
coordinates(meuse) = ~x+y
v = variogram(log(zinc)~1, meuse)
v.m = fit.variogram(v, vgm(1, "Sph", 500, 1))
v[1, "gamma"] # your estimator:
plot(v, v.m)
v.m$psill[1] / sum(v.m$psill) # the usual estimator:

Only when you have plenty of measurements at very short distances
apart, I can imagine you would use the sample variogram (v) value.

> Thanks very much for your attention. 
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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