.. py:function:: interpolate(values, in_grid=None, out_grid=None, matrix_source=None, method='linear')
Interpolate the ``values`` from the ``in_grid`` onto the ``out_grid``.
:param values: the input data. It can be an ndarray or an earthkit-data GRIB :xref:`fieldlist`. ndarrays are assumed to be defined on the ``in_grid``. The :xref:`fieldlist` support requires :xref:`earthkit-data` >= 0.6.0 and only works when the ``out_grid`` is a regular latitude-longitude grid.
:type values: ndarray, :xref:`fieldlist`
:param in_grid: the :ref:`gridspec <gridspec>` describing the grid that ``values`` are defined on. When ``values`` is a :xref:`fieldlist` the input grid is automatically detected and ``in_grid`` cannot be specified.
:type in_grid: dict
:param out_grid: the :ref:`gridspec <gridspec>` describing the target grid that ``values`` will be interpolated onto
:type out_grid: dict
:param method: the interpolation method. Possible values are ``linear`` and ``nearest-neighbour``. For ``nearest-neighbour`` the following aliases are also supported: ``nn``, ``nearest-neighbor``.
:type method: str
:param matrix_source: (experimental) the location of a user specified pre-generated matrix inventory. When it is None the default matrix inventory hosted on an ECMWF download server is used.
:type matrix_source: str, None
:return: The same type of data as ``values`` containing the interpolated values.
:rtype: ndarray, :xref:`fieldlist`
:raises ValueError: if a pre-generated interpolation matrix is not available
:raises ValueError: if ``in_grid`` is specified for a :xref:`fieldlist` input
The interpolation only works when a pre-generated interpolation matrix is available for the given ``in_grid``, ``out_grid`` and ``method`` combination.
When ``matrix_source`` is None (default) the interpolation matrix is automatically downloaded and stored in a local cache and when it is needed again the cached version is used.
Once the matrix is available the interpolation is performed by multiplying the ``values`` vector with it.
Note
Please see the :ref:`inventory <matrix_inventory>` for the list of available matrices.