@BeginModule
@NewPage
@Name      = Intgrid
@Title     = Grid interpolation
@Section   = Interpolation
@Class     = Interpolation
@Arguments = ifile ofile
@Operators = interpolate


@BeginDescription
This module contains operators to interpolate all input fields to a new grid.
All interpolation methods in this module only work on regular lon/lat grids.
@EndDescription
@EndModule


@BeginOperator_interpolate
@Title     = PINGO grid interpolation
@Parameter = grid

@BeginDescription
This is the grid interpolation from PINGO.
@IfDoc
The base of the interpolation is an underlying continuous field
which is constructed in the following way: \\
For two neighboured longitudes @math{x_1} and @math{x_2} and two neighboured 
latitudes @math{y_1} and @math{y_2} of the input grid every point at
longitude @math{x} and latidude @math{y} with @math{x_1 \le x \le x_2}
and @math{y_1 \le y \le y_2} is assigned the value
\begin{eqnarray*}
a & = & a_{11} + (a_{21}-a_{11}) \frac{x-x_1}{x_2-x_1} + (a_{12}-a_{11}) \frac{y-y_1}{y_2-y_1} \\
  &   & {}+ (a_{22}-a_{21}-a_{12}+a_{11}) \frac{(x-x_1)(y-y_1)}{(x_2-x_1)(y_2-y_1)}
\end{eqnarray*}
where $a_{ij}$ is the value at longitude @math{x_i} and latitude
@math{y_j}. If one out of the four values $a_{11}$, $a_{12}$, $a_{21}$, $a_{22}$ is the
missing value then @math{a} is also the missing value.
Afterwards the underlying continuous field is expanded by a half 
mesh width.
@EndIfDoc
For a detailed description of this interpolation method see @cite{PINGO}.
@EndDescription
@EndOperator


@BeginOperator_intgridbil
@Title     = Bilinear grid interpolation
@Parameter = grid

@BeginDescription
Performs a bilinear interpolation on all input fields. This implementation
is faster than @oper{remapbil}. The complete target grid have to be inside
the source grid. Missing values are not supported yet!
@EndDescription
@EndOperator


@BeginOperator_intpoint
@Title     = Point interpolation
@Parameter = long lat

@BeginDescription
Linear point interpolation.
@EndDescription
%@Item = long
%FLOAT  Longitude of the point
%@Item = lat
%FLOAT  Latitude of the point
@EndOperator


@BeginParameter
@Item = grid
STRING  Target grid description file or name
@EndParameter


@BeginExample
Say @file{ifile} contains fields on a regular lon/lat grid.
To interpolate all fields bilinear to a T42 Gaussian grid use:
@BeginVerbatim
   cdo intgridbil,t42grid ifile ofile
@EndVerbatim
@EndExample
