\Header{expcov}{Spatial Covariance Functions}
\alias{gaucov}{expcov}
\alias{sphercov}{expcov}
\keyword{spatial}{expcov}
\begin{Description}\relax
Spatial covariance functions for use with \code{surf.gls}.
\end{Description}
\begin{Usage}
\begin{verbatim}
expcov(r, d, alpha = 0, se = 1)
gaucov(r, d, alpha = 0, se = 1)
sphercov(r, d, alpha = 0, se = 1, D = 2)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{r}] vector of distances at which to evaluate the covariance

\item[\code{d}] range parameter

\item[\code{alpha}] proportion of nugget effect

\item[\code{se}] standard deviation at distance zero

\item[\code{D}] dimension of spheres.

\end{ldescription}
\end{Arguments}
\begin{Value}
vector of covariance values.\end{Value}
\begin{References}\relax
Ripley, B. D. (1981) \emph{Spatial Statistics.} Wiley.

Venables, W. N. and Ripley, B. D. (2002)
\emph{Modern Applied Statistics with S.} Fourth edition.  Springer.\end{References}
\begin{SeeAlso}\relax
\code{\Link{surf.gls}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
data(topo, package="MASS")
topo.kr <- surf.ls(2, topo)
correlogram(topo.kr, 25)
d <- seq(0, 7, 0.1)
lines(d, expcov(d, 0.7))
\end{ExampleCode}
\end{Examples}

