[R-sig-Geo] Fwd: Variogram in spatial package

[Roger Bivand]

On Fri, 26 Jan 2007, Hywel Jones wrote:

> A resubmission, as no one has replied and I still have
> the problem.
> --- Hywel Jones <hywelm.jones at talk21.com> wrote:
> 
> > Date: Wed, 17 Jan 2007 09:58:10 +0000 (GMT)
> > From: Hywel Jones <hywelm.jones at talk21.com>
> > Subject: Variogram in spatial package
> > To: r-sig-geo at stat.math.ethz.ch
> > 
> > The help page for variogram in the spatial package
> > leaves my slightly uncertain about a few things. I'd
> > be grateful for confirmation of my understanding.

The spatial package is described in more detail in Venables, W. N. and
Ripley, B. D. (2002) _Modern Applied Statistics with S._ Fourth edition.  
Springer, as the help page says.

> > 
> > If I fit a trend surface using surf.ls, and then set
> > the krig parameter to use that object in the
> > variogram
> > function, I'm assuming that the variogram produced
> > is
> > calculated for the residuals contained in the trend
> > surface object. Is that right?

Yes, I believe so.

> > 
> > I'm afraid I don't have the references to check the
> > following either. Using notation of Cressie, am I
> > right in thinking that the y co-ordinate of the
> > variogram corresponds to gamma (or 2x gamma)? And x:
> > is that h? 

The documentation is the source code. Alternatively run the same data
through similar functions in the gstat, geoR, or fields packages to
cross-check?  My reading of MASS is that the plot is of the semi-variogram
with distances in the original scaling.

> > 
> > And then, how does h correspond to my original data?
> > i.e. do I interpret it as distance calculated with
> > the
> > original x and y submitted to surf.ls, or as
> > distance
> > calculated with the rescaled x and y used within
> > surf.ls (I understand that the internals rescale x
> > and
> > y to -1:1).
> > 
> > I'd actually like to check for isotropy before using
> > this variogram function. Any suggestions as to
> > functions I might use?

I don't think anisotropy is available in the spatial package, perhaps try 
one of the other packages?

> > 
> > Thanks in advance.
> > 
> > Hywel
> > 
> > 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no