nugget in variogram

vgm(1, "Exp", 300)

vgm(1, "Exp", 300, 1)

Hello
I am really glad of your answer, but for me it does not work as I would like
So, my data are called cc (is a subset of a bigger spatialdataframe) and the variogram would be
varcc=variogram(pf1_3atm~1,cc, width=500,cutoff=10000) 
 
Then I fix the sill and the range of the variogram
inisill=var(cc at data$pf1_3atm,na.rm=T) 
iniran=var$dist[varcc$gamma>0.9*inisill][1]
varcc.fit=fit.variogram(varcc,vgm(psill=inisill, "Exp", iniran)) # I can not fix the nugget because the output gives errors ("Singular model")
plot(varcc,varcc.fit)

varcc[1,"gamma"]  this would be the nugget (where the model cuts the axis)

[1] 11.97
The problem in the indicator you suggest is:
varcc.fit$psill[1] / sum(varcc.fit$psill)= 1, because  varcc.fit$psill[1] = sum(varcc.fit$psill)
as the output of varcc.fit gives:
varcc.fit
  model    psill    range
1   Exp 41.46455 1297.040

I have tried (don't know if it is correct):

 varcc[1,"gamma"]/sum(varcc.fit$psill)

[1] 0.2886803
(and this would mean that the spatial correlation is weak because we have a relative big nugget effect)
Can this be done? 
Thanks a lot again for your attention.

Date: Thu, 30 Sep 2010 18:39:22 +0200
From: edzer.pebesma at uni-muenster.de
To: r-sig-geo at stat.math.ethz.ch
Subject: Re: [R-sig-Geo] nugget in variogram



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 M?nster
Weseler Stra?e 253, 48151 M?nster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de