- value
An optional vector of parameter values, with serves as initial values or as fixed values depending on the fixed argument.
It has either two or three elements, depending on the nugget argument.
If nugget is FALSE, value should have
two elements, corresponding to the "range" and the "smoothness" \(\nu\) of the
Matérn correlation structure. If value has zero length,
the default is a range of 90% of the minimum distance and a
smoothness of 0.5 (exponential correlation).
Warning: the range parameter used in corSpatial objects is the inverse of the scale
parameter used in MaternCorr and thus they have opposite meaning despite both being denoted \(\rho\) elsewhere in this package or in nlme literature.
If nugget is TRUE, meaning that a nugget effect
is present, value can contain two or three elements, the first
two as above, the third being the "nugget effect" (one minus the
correlation between two observations taken arbitrarily close
together). If value has length zero or two, the nugget defaults to 0.1.
The range and smoothness must be greater than zero and the nugget must be
between zero and one.
- form
(Pasted from corSpatial) a one sided formula of the form ~ S1+...+Sp, or
~ S1+...+Sp | g, specifying spatial covariates S1
through Sp and, optionally, a grouping factor g.
When a grouping factor is present in 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 ~ 1, which corresponds
to using the order of the observations in the data as a covariate,
and no groups.
- nugget
an optional logical value indicating whether a nugget
effect is present. Defaults to FALSE.
- nuScaled
If nuScaled is set to TRUE the "range" parameter
\(\rho\) is divided by \(2 \sqrt\nu\). With this option and
for large values of \(\nu\), corMatern reproduces the
calculation of corGaus. Defaults to FALSE, in which
case the function compares to corGaus with range parameter
\(2(\sqrt\nu)\rho\) when \(\nu\) is large.
- metric
(Pasted from corSpatial) an optional character string specifying the distance
metric to be used. The currently available options are
"euclidean" for the root sum-of-squares of distances;
"maximum" for the maximum difference; and "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 "euclidean".
- 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 FALSE, in which case
the coefficients are allowed to vary.