# approx3d: Trilinear interpolation in a 3D array In oce: Analysis of Oceanographic Data

 approx3d R Documentation

## Trilinear interpolation in a 3D array

### Description

Interpolate within a 3D array, using the trilinear approximation.

### Usage

```approx3d(x, y, z, f, xout, yout, zout)
```

### Arguments

 `x` vector of x values for grid (must be equi-spaced) `y` vector of y values for grid (must be equi-spaced) `z` vector of z values for grid (must be equi-spaced) `f` matrix of rank 3, with the gridded values mapping to the `x` values (first index of `f`), etc. `xout` vector of x values for output. `yout` vector of y values for output (length must match that of `xout`). `zout` vector of z values for output (length must match that of `xout`).

### Details

Trilinear interpolation is used to interpolate within the `f` array, for those (`xout`, `yout` and `zout`) triplets that are inside the region specified by `x`, `y` and `z`. Triplets that lie outside the range of `x`, `y` or `z` result in `NA` values.

### Value

A vector of interpolated values (or `NA` values), with length matching that of `xout`.

### Author(s)

Dan Kelley and Clark Richards

### Examples

```## set up a grid
library(oce)
n <- 5
x <- seq(0, 1, length.out=n)
y <- seq(0, 1, length.out=n)
z <- seq(0, 1, length.out=n)
f <- array(1:n^3, dim=c(length(x), length(y), length(z)))
## interpolate along a diagonal line
m <- 100
xout <- seq(0, 1, length.out=m)
yout <- seq(0, 1, length.out=m)
zout <- seq(0, 1, length.out=m)
approx <- approx3d(x, y, z, f, xout, yout, zout)
## graph the results
plot(xout, approx, type='l')
points(xout, f[1, 1, 1])
points(xout[m], f[n,n,n])
```

oce documentation built on July 6, 2022, 1:06 a.m.