Thanks for your advice. Let me re-phrase my question with a reproducible
example. Same message was also posted to the mixed-model list. Sorry for
the cross-posting.

The example and data are from UCLA's R FAQ webpage. The link for the
webpage is:  http://www.ats.ucla.edu/stat/r/faq/spatial_regression.htm

First, following the instructions on the webpage, I did the following：

install.packages("nlme")
library(nlme)

spdata <- read.table("http://www.ats.ucla.edu/stat/R/faq/thick.csv", header
= T, sep = ",")
dummy <- rep(1, 75)
spdata <- cbind(spdata, dummy)

### estimated the null model ###

soil.model <- lme(fixed = thick ~ soil, data = spdata, random = ~ 1 |
dummy, method = "ML")
summary(soil.model)
plot(Variogram(soil.model,form=~north+east))

### then updated the model by the spatial correlation structure ####

soil.gaus <- update(soil.model, correlation=corGaus(1,form=~north+east))
summary(soil.gaus)
plot(Variogram(soil.gaus,form=~north+east))

*My first question is*: the variogram of soil.gaus indicates a range of 40,
but the estimated range is 20.43725. Does this mean the model "soil.gaus"
failed?

Next, I define the range and nugget manually as below. The variogram of
soil.model indicates a range at 40 and nugget at about 0.1.

soil.gaus.1 <- update(soil.model,
correlation=corGaus(c(40,0.1),form=~north+east,nugget=T))
summary(soil.gaus.1)
plot(Variogram(soil.gaus.1,form=~north+east))

The estimated range looks right (very close to 40). But the estimated
nugget and variogram plot still look weird. *So my question is*: what I can
do next step to improve the model?

Any advice would be greatly appreciated!

Best
Gary

On Sat, Jun 16, 2012 at 2:23 AM, Edzer Pebesma <
edzer.pebesma@uni-muenster.de> wrote:

> It would have helped us when you would have provided an example that was
> reproducible for readers of this list.
>
> I tried some things out, along that way, but didn't manage. Maybe you
> could try the mixed-models list:
>
> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>
> If you find out how these things work, feedback to this list would be
> appreciated.
>
> On 06/16/2012 04:52 AM, wudadan wrote:
> > Dear R users,
> >
> > I need some help in understanding the spatial correlation structure. Is
> > there a document or paper that explains how to use spatial correlation
> > structure such as "corExp"?
> >
> > Here is the problem I run into. With the spatial correlation structure:
> >
> >  "correlation = corExp(c(10,0.8),form = ~ north + east)"
> >
> >  I tried to define the range and nugget as 10 miles and 0.8,
> respectively.
>
> the help pages here suggest you'd have to add nugget=TRUE to the corExp
> arguments.
>
> > However, I got the error message "Error in
> initialize.corSpatial....Initial
> > value for corSpatial parameters of wrong dimension". Can anyone tell me
> what
> > this means?
> >
> > I then tried the script:
> >
> > "correlation = corExp(10,form = ~ north + east)"
> >
> > It worked. However, it seems the range "10" was not used because after I
> > changed it to "3", I got exactly the same estimation results. I'm
> wondering
> > what the right way to do this.
> >
> > Any advice is appreciated!
> >
> > Best
> > Gary
> >
> >
> >
> >
> >
> >
> >
> > --
> > View this message in context:
> http://r-sig-geo.2731867.n2.nabble.com/Need-help-understanding-the-spatial-correlation-structure-applied-in-lme-tp6049193p7580220.html
> > Sent from the R-sig-geo mailing list archive at Nabble.com.
> >
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo@r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
> --
> 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@wwu.de
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo@r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>

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