@staticmethod
def _interp_spatial(a):
    """
        Helper function for spatial interpolation.
        """
    import scipy.interpolate as si
    t, d, points, msk, grid_lat, grid_lon, method = a
    new_data = si.griddata(points, d[~msk], (grid_lat, grid_lon), method=method
        )
    return t, new_data
