R: Covariance functions

covmat {spam} | R Documentation

## Covariance functions

### Description

Evaluate a covariance function.

### Usage

    covmat(h, theta, ... , type="sph")

    cov.exp(h, theta, ... , eps= getOption("spam.eps"))
    cov.sph(h, theta, ... , eps= getOption("spam.eps"))
    cov.nug(h, theta, ... , eps= getOption("spam.eps"))
    cov.wu1(h, theta, ... , eps= getOption("spam.eps"))
    cov.wu2(h, theta, ... , eps= getOption("spam.eps"))
    cov.wu3(h, theta, ... , eps= getOption("spam.eps"))
    cov.wend1(h, theta, ... , eps= getOption("spam.eps"))
    cov.wend2(h, theta, ... , eps= getOption("spam.eps"))
    cov.mat(h, theta, ... , eps= getOption("spam.eps"))

### Arguments

`h` | object containing the lags.
`theta` | parameter of the covariance function, see ‘Details’.
`type` | covariance function specification.
`...` | arguments passed from other methods.
`eps` | tolerance level.

### Details

`covmat` is a wrapper that calls the other functions according to the argument `type`. The nomenclature is similar to `premat`
The parametrization is (range, sill, [smoothness], nugget), where only the range needs to be specified. Default values are (1,[1],0). In case of negative parameter values, a warning is issued and the absolute value is retained. Although more cryptic, having all arguments as a single vector simplifies optimization with `optim`.
Currently, the functions distinguish between a sparse `spam` object `h` and any other numeric type. In the future, this might change and appropriate methods will be implemented.

### Value

Covariance function evaluated on `h`.

### Author(s)

Reinhard Furrer

### References

Any classical book about geostatistics.

### See Also

`precmat`.

### Examples

    locs <- cbind(runif(10),runif(10))
    h <- nearest.dist(locs, delta=.3)
    Sigma <- cov.sph(h, c(.3, 1, .1))

    ## Not run:
    h <- seq(0, to=1, length.out=100)
    plot( h, cov.exp(h, c(1/3,1)), type='l', ylim=c(0,1))
    type <- c("sph","wendland1","wendland2","wu1","wu2","wu3")
    for (i in 1:6)
      lines( h, covmat(h, 1, type=type[i]), col=i+1)
    legend('topright',legend=type, col=2:7, lty=1)

    ## End(Not run)

[Package spam version 2.1-1 Index]