Nugget from variogram.fit vs GLS

Hi All,

I'm verifying coefficients generated with GSTAT kriging with external 
drift using GLS. The coefficients match absolutely perfectly. The range 
also matches perfectly. But there is a big difference in what GLS gives 
as the output for the nugget. Can anyone explain why this is?

Zev

###### KED

# here's where I fit the variogram and the fit output

v.fit = fit.variogram(v, vgm(1.5, "Exp", 20, 0.5))

 > v.fit
   model    psill     range
1   Nug 1.302310 0.0000000
2   Exp 1.413246 0.8098587

# the fit is used in a KED model
g = gstat(formula=formula(MODEL), data = so2, model = v.fit)

# here I get the coefs (these coefs match those from GLS below perfectly)
UKcoef<-predict(g, lattice, BLUE=c(TRUE,TRUE))

###### GLS

# use the same parameters as above and fix them

nug<-v.fit[1,2] # this is 1.302
sill<-v.fit[1,2]+v.fit[2,2]
range<-v.fit[2,3]

theGLS<-gls(formula(so2FinalAll), data=so2, na.action=na.omit, 
correlation=corExp(c(range, nug/sill ),
     form=~X+Y, nugget=TRUE,fixed=TRUE), method="ML")

 > summary(theGLS)
Generalized least squares fit by maximum likelihood
   Model: formula(so2FinalAll)
   Data: so2
        AIC      BIC    logLik
   578.6551 593.7083 -284.3275

Correlation Structure: Exponential spatial correlation
  Formula: ~X + Y
  Parameter estimate(s):
     range    nugget
0.8098587 0.4795740

# note the nugget of 0.479 vs the 1.302 above.

Zev Ross
ZevRoss Spatial Analysis
120 N Aurora, Suite 3A
Ithaca, NY 14850
607-277-0004 (phone)
866-877-3690 (fax, toll-free)
zev at zevross.com