corSpatial {nlme} | R Documentation

## Spatial Correlation Structure

### Description

This function is a constructor for the `corSpatial` class, representing a spatial correlation structure. This class is "virtual", having four "real" classes, corresponding to specific spatial correlation structures, associated with it: `corExp`, `corGaus`, `corLin`, `corRatio`, and `corSpher`. The returned object will inherit from one of these "real" classes, determined by the `type` argument, and from the "virtual" `corSpatial` class. Objects created using this constructor must later be initialized using the appropriate `Initialize` method.

### Usage

    corSpatial(value, form, nugget, type, metric, fixed)

### Arguments

`value`

an optional vector with the parameter values in constrained form. If `nugget` is `FALSE`, `value` can have only one element, corresponding to the "range" of the spatial correlation structure, which must be greater than zero. If `nugget` is `TRUE`, meaning that a nugget effect is present, `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 `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 `object` is initialized.

`form`

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`.

`type`

an optional character string specifying the desired type of correlation structure. Available types include `"spherical"`, `"exponential"`, `"gaussian"`, `"linear"`, and `"rational"`. See the documentation on the functions `corSpher`, `corExp`, `corGaus`, `corLin`, and `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 `"spherical"`.

`metric` | 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.

### Value

an object of class determined by the `type` argument and also inheriting from class `corSpatial`, representing a spatial correlation structure.

### Author(s)

Jose Pinheiro Jose.Pinheiro@pharma.novartis.com and Douglas Bates bates@stat.wisc.edu

### References

Cressie, N.A.C. (1993), "Statistics for Spatial Data", J. Wiley & Sons.

Venables, W.N. and Ripley, B.D. (1997) "Modern Applied Statistics with S-plus", 2nd Edition, Springer-Verlag.

Littel, Milliken, Stroup, and Wolfinger (1996) "SAS Systems for Mixed Models", SAS Institute.

### See Also

`corExp`, `corGaus`, `corLin`, `corRatio`, `corSpher`, `Initialize.corStruct`, `summary.corStruct`, `dist`

### Examples

    sp1 <- corSpatial(form = ~ x + y + z, type = "g", metric = "man")

[Package nlme version 3.1-93 Index]