R: Gridded Bivariate Interpolation for Irregular Data

interp.old {akima} | R Documentation

## Gridded Bivariate Interpolation for Irregular Data

### Description

These functions implement bivariate interpolation onto a grid for irregularly spaced input data. These functions are only for backward compatibility, use `interp` instead.

### Usage

    interp.old(x, y, z, xo= seq(min(x), max(x), length = 40),
               yo=seq(min(y), max(y), length = 40), ncp = 0,
               extrap=FALSE, duplicate = "error", dupfun = NULL)
    interp.new(x, y, z, xo = seq(min(x), max(x), length = 40),
               yo = seq(min(y), max(y), length = 40), linear = FALSE,
               ncp = NULL, extrap=FALSE, duplicate = "error", dupfun = NULL)

### Arguments

`x` | vector of x-coordinates of data points or a `SpatialPointsDataFrame` object. Missing values are not accepted.
`y` | vector of y-coordinates of data points. Missing values are not accepted. If left as NULL indicates that `x` should be a `SpatialPointsDataFrame` and `z` names the variable of interest in this dataframe.
`z` | vector of z-coordinates of data points or a character variable naming the variable of interest in the `SpatialPointsDataFrame` `x`. Missing values are not accepted. `x`, `y`, and `z` must be the same length (execpt if `x` is a `SpatialPointsDataFrame`) and may contain no fewer than four points. The points of `x` and `y` cannot be collinear, i.e, they cannot fall on the same line (two vectors `x` and `y` such that `y = ax + b` for some `a`, `b` will not be accepted). `interp` is meant for cases in which you have `x`, `y` values scattered over a plane and a `z` value for each. If, instead, you are trying to evaluate a mathematical function, or get a graphical interpretation of relationships that can be described by a polynomial, try `outer()`.
`xo` | vector of x-coordinates of output grid. The default is 40 points evenly spaced over the range of `x`. If extrapolation is not being used (`extrap=FALSE`, the default), `xo` should have a range that is close to or inside of the range of `x` for the results to be meaningful.
`yo` | vector of y-coordinates of output grid; analogous to `xo`, see above.
`linear` | logical – indicating wether linear or spline interpolation should be used. supersedes old `ncp` parameter
`ncp` | deprecated, use parameter `linear`. Now only used by `interp.old()`. old meaning was: number of additional points to be used in computing partial derivatives at each data point. `ncp` must be either `0` (partial derivatives are not used), or at least 2 but smaller than the number of data points (and smaller than 25).
`extrap` | logical flag: should extrapolation be used outside of the convex hull determined by the data points?
`duplicate` | character string indicating how to handle duplicate data points. Possible values are `"error"` produces an error message, `"strip"` remove duplicate z values, `"mean"`,`"median"`,`"user"` calculate mean , median or user defined function (`dupfun`) of duplicate z values.
`dupfun` | a function, applied to duplicate points if `duplicate= "user"`.

### Details

see `interp`

### Value

list with 3 components:

`x,y` | vectors of x- and y- coordinates of output grid, the same as the input argument `xo`, or `yo`, if present. Otherwise, their default, a vector 40 points evenly spaced over the range of the input `x`.
`z` | matrix of fitted z-values. The value `z[i,j]` is computed at the x,y point `xo[i], yo[j]`. `z` has dimensions `length(xo)` times `length(yo)`.

If input is a `SpatialPointsDataFrame` a `SpatialPixelssDataFrame` is returned.

### Note

`interp.new` is deprecated and `interp.old` will soon be deprecated.

### References

Akima, H. (1978). A Method of Bivariate Interpolation and Smooth Surface Fitting for Irregularly Distributed Data Points. ACM Transactions on Mathematical Software 4, 148-164.

Akima, H. (1996). Algorithm 761: scattered-data surface fitting that has the accuracy of a cubic polynomial. ACM Transactions on Mathematical Software 22, 362–371.

### See Also

`contour`, `image`, `approx`, `spline`, `aspline`, `outer`, `expand.grid`.

[Package akima version 0.6-2 Index]