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% File nlme/man/corSpatial.Rd
% Part of the nlme package for R
% Distributed under GPL 2 or later: see nlme/LICENCE.note
\name{corSpatial}
\title{Spatial Correlation Structure}
\usage{
corSpatial(value, form, nugget, type, metric, fixed)
}
\alias{corSpatial}
\arguments{
\item{value}{an optional vector with the parameter values in
constrained form. If \code{nugget} is \code{FALSE}, \code{value} can
have only one element, corresponding to the "range" of the
spatial correlation structure, which must be greater than
zero. If \code{nugget} is \code{TRUE}, meaning that a nugget effect
is present, \code{value} can contain one or two elements, the first
being the "range" and the second the "nugget effect" (one minus the
correlation between two observations taken arbitrarily close
together); the first must be greater than zero and the second must be
between zero and one. Defaults to \code{numeric(0)}, which results in
a range of 90\% of the minimum distance and a nugget effect of 0.1
being assigned to the parameters when \code{object} is initialized.}
\item{form}{a one sided formula of the form \code{~ S1+...+Sp}, or
\code{~ S1+...+Sp | g}, specifying spatial covariates \code{S1}
through \code{Sp} and, optionally, a grouping factor \code{g}.
When a grouping factor is present in \code{form}, the correlation
structure is assumed to apply only to observations within the same
grouping level; observations with different grouping levels are
assumed to be uncorrelated. Defaults to \code{~ 1}, which corresponds
to using the order of the observations in the data as a covariate,
and no groups.}
\item{nugget}{an optional logical value indicating whether a nugget
effect is present. Defaults to \code{FALSE}.}
\item{type}{an optional character string specifying the desired type of
correlation structure. Available types include \code{"spherical"},
\code{"exponential"}, \code{"gaussian"}, \code{"linear"}, and
\code{"rational"}. See the documentation on the functions
\code{corSpher}, \code{corExp}, \code{corGaus}, \code{corLin}, and
\code{corRatio} for a description of these correlation
structures. Partial matching of arguments is used, so only the first
character needs to be provided.Defaults to \code{"spherical"}.}
\item{metric}{an optional character string specifying the distance
metric to be used. The currently available options are
\code{"euclidean"} for the root sum-of-squares of distances;
\code{"maximum"} for the maximum difference; and \code{"manhattan"}
for the sum of the absolute differences. Partial matching of
arguments is used, so only the first three characters need to be
provided. Defaults to \code{"euclidean"}.}
\item{fixed}{an optional logical value indicating whether the
coefficients should be allowed to vary in the optimization, or kept
fixed at their initial value. Defaults to \code{FALSE}, in which case
the coefficients are allowed to vary.}
}
\description{
This function is a constructor for the \code{corSpatial} class,
representing a spatial correlation structure. This class is "virtual",
having four "real" classes, corresponding to specific spatial
correlation structures, associated with it: \code{corExp},
\code{corGaus}, \code{corLin}, \code{corRatio}, and
\code{corSpher}. The returned object will inherit from one of these
"real" classes, determined by the \code{type} argument, and from the
"virtual" \code{corSpatial} class. Objects created using this
constructor must later be initialized using the appropriate
\code{Initialize} method.
}
\value{
an object of class determined by the \code{type} argument and also
inheriting from class \code{corSpatial}, representing a spatial
correlation structure.
}
\references{
Cressie, N.A.C. (1993), "Statistics for Spatial Data", J. Wiley & Sons.
Venables, W.N. and Ripley, B.D. (2002) "Modern Applied Statistics with
S", 4th Edition, Springer-Verlag.
Littel, Milliken, Stroup, and Wolfinger (1996) "SAS Systems for Mixed
Models", SAS Institute.
}
\author{José Pinheiro and Douglas Bates \email{bates@stat.wisc.edu}}
\seealso{
\code{\link{corExp}},
\code{\link{corGaus}},
\code{\link{corLin}},
\code{\link{corRatio}},
\code{\link{corSpher}},
\code{\link{Initialize.corStruct}},
\code{\link{summary.corStruct}},
\code{\link{dist}}
}
\examples{
sp1 <- corSpatial(form = ~ x + y + z, type = "g", metric = "man")
}
\keyword{models}
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