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# g2gsh

Interpolates a scalar quantity from one Gaussian grid to another (with optional truncation).

## Prototype

```	function g2gsh (
grid        : numeric,
outdims [2] : byte, short, integer or long,
twave   [1] : integer
)

return_val  :  float or double
```

## Arguments

grid

An array of 2 or more dimensions whose rightmost two dimensions must be latitude x longitude. The values must be in ascending latitude order.

outdims

An array indicating the dimensions of the rightmost two dimensions of the output grid (outdims[0] = nlatb, outdims[1] = nlonb).

As of version 6.0.0, this can be of type byte, short, integer or long.

twave

A scalar integer indicating the optional wave truncation:

twave = 0 => exact interpolation
twave > 0 => exact interpolation and triangular truncation at twave
twave < 0 => exact interpolation, triangular truncation at twave and spectral coefficient tapering (the effect is to smooth the data)

## Return value

The return array's dimensions are the same as grid's dimensions, except the rightmost two dimensions, nlata and nlona, are replaced by nlatb and nlonb. The type will be double if the input is double, and float otherwise.

## Description

g2gsh interpolates a scalar quantity from one Gaussian grid to another (with optional truncation) using spherical harmonics (via the Spherepack code). Values will be in ascending latitude order.

If missing values are encountered in a 2D subsection of the input array, then no interpolation will be performed on that 2D array, and the corresponding 2D subsection of the return array will be filled with missing values.

Arrays which have dimensions [...] x nlata x nlona should not include the cyclic (wrap-around) points when invoking this function. For example, if an array x has dimensions nlata = 64 and nlona = 129, where the "129" represents the cyclic point, then the user should pass the data to g2gsh via:

```  z = g2gsh ( x(...,0:nlona-2), twave )  ; does not include cyclic points
```

This function can only be used on grids that span the globe. It cannot be used for limited area grids.

Please read the caveats that you should be aware of when using spherical harmonic regridding.

Use the g2gsh_Wrap function if metadata retention is desired. The interface is identical.

## Examples

Example 1

Assume x is dimensioned nt x nlat x nlon. The code below interpolates from a Gaussian T63 grid (nlat=96,nlon=192) to a Gaussian T42 grid (jlat=64,ilon=128) with triangular truncation at wave 42:

```  X = g2gsh (x, (/jlat,ilon/), 42) ; ==> X(nt,jlat,ilon)

; Use g2gsh_Wrap if metadata retention is desired
; X = g2gsh_Wrap (x, (/jlat,ilon/), 42) ; ==> X(nt,jlat,ilon)
```

## Errors

ier is equal to:

4 or 10 if nlona is less than 4
5 or 10 if nlata is less than 3
8 if nlonb is less than 4
9 if nlatb is less than 3