ctdDecimate {oce} | R Documentation

## Decimate a CTD profile

### Description

Interpolate a CTD profile to specified pressure values.

### Usage

    ctdDecimate(x, p=1, method="boxcar", e=1.5, debug=getOption("oceDebug"))

### Arguments

`x` | a `ctd` object, e.g. as read by `read.ctd`.
`p` | pressure increment, or vector of pressures. In the first case, pressures from 0dbar to the rounded maximum pressure are used, incrementing by `p` dbars. If a vector of pressures is given, interpolation is done to these pressures.
`method` | the method to be used for calculating decimated values. This may be a function or a string naming a built-in method. The built-in methods are `"boxcar"` (based on a local average), `"approx"` (based on linear interpolation between neighboring points), `"lm"` (based on local regression, with `e` setting the size of the local region), `"rr"` (for the Reineger and Ross method, carried out with `oce.approx`) and `"unesco"` (for the UNESCO method, carried out with. `oce.approx`. If `method` is a function, then it must take three arguments, the first being pressure, the second being an arbitrary variable in another column of the data, and the third being a vector of target pressures at which the calculation is carried out, and the return value must be a vector. See “Examples”.
`e` | is an expansion coefficient used to calculate the local neighbourhoods for the `"boxcar"` and `"lm"` methods. If `e=1`, then the neighbourhood for the i-th pressure extends from the (`i-1`)-th pressure to the (`i+1`)-th pressure. At the endpoints it is assumed that the outside bin is of the same pressure range as the first inside bin. For other values of `e`, the neighbourhood is expanded linearly in each direction. If the `"lm"` method produces warnings about "prediction from a rank-deficient fit", a larger value of `"e"` should be used.
`debug` | a Boolean, set to `TRUE` to debug the reading process.

### Details

The `"approx"` method is best for bottle data, in which the usual task is to interpolate from a coarse sampling grid to a finer one. For CTD data, the `"boxcar"` method is the more common choice, because the task is normally to sub-sample, and some degree of smoothing is usually desired. (The `"lm"` method is quite slow, and the results are similar to those of the boxcar method.)

NB. A sort of numerical cabeling effect can result from this procedure, but it can be avoided as follows

    xd <- ctdDecimate(x)
    xd[["sigmaTheta"]] <- swSigmaTheta(xd[["salinity"]],xd[["temperature"]],xd[["pressure"]])

### Value

An object of `class` `"ctd"`, with pressures that are as set by the `"p"` parameter and all other properties modified appropriately.

### Author(s)

Dan Kelley

### References

R.F. Reiniger and C.K. Ross, 1968. A method of interpolation with application to oceanographic data. Deep Sea Research, 15, 185-193.

### See Also

The documentation for `ctd-class` explains the structure of CTD objects, and also outlines the other functions dealing with them.

### Examples

    library(oce)
    data(ctd)
    plotProfile(ctd, "salinity", ylim=c(10, 0))
    p <- seq(0, 45, 1)
    ctd2 <- ctdDecimate(ctd, p=p)
    lines(ctd2[["salinity"]], ctd2[["pressure"]], col="blue")
    p <- seq(0, 45, 1)
    ctd3 <- ctdDecimate(ctd, p=p, method=function(x,y,xout)
                        predict(smooth.spline(x, y, df=30), p)$y)
    lines(ctd3[["salinity"]], ctd3[["pressure"]], col="red")

[Package oce version 0.9-18 Index]