# intrplon

Interpolate longitude at given latitude

## Syntax

```newlon = intrplon(lat,lon,newlat) newlon = intrplon(lat,lon,newlat,method) newlon = intrplon(lat,lon,newlat,method,units) ```

## Description

`newlon = intrplon(lat,lon,newlat) ` returns an interpolated longitude, `newlon`, corresponding to a latitude `newlat`. `lat` must be a monotonic vector of longitude values. `lon` is a vector of the longitude values paired with each entry in `lat`.

`newlon = intrplon(lat,lon,newlat,method)` specifies the `method` of interpolation employed, listed in the table below.

MethodDescription
`'linear'`Linear, or Cartesian, interpolation (default)
`'pchip'`Piecewise cubic Hermite interpolation
`'rh'`Returns interpolated points that lie on rhumb lines between input data
`'gc'`Returns interpolated points that lie on great circles between input data

`newlon = intrplon(lat,lon,newlat,method,units)` specifies the units used, where `units` is any valid angle units string scalar or character vector. The default is `'degrees'`.

The function `intrplon` is a geographic data analogy of the MATLAB® function `interp1`.

## Examples

Compare the results of the various methods:

```long = [25 45]; lat = [30 60]; newlon = intrplon(lat,long,45,'linear') newlon = 35 newlon = intrplon(lat,long,45,'rh') newlon = 33.6515 newlon = intrplon(lat,long,45,'gc') newlon = 32.0526```

## Tips

There are separate functions for interpolating latitudes and longitudes, for although the cases are identical when using those methods supported by `interp1`, when latitudes and longitudes are treated like the spherical angles they are (using `'rh'` or `'gc'`), the results are different. Compare the previous example to the example under `intrplat`, which reverses the values of latitude and longitude.

## Version History

Introduced before R2006a