def _interp(self, data):
        """The interpolation method implemented here is a kind of a billinear
        interpolation. The input *data* field is first interpolated along the 
        rows and subsequently along its columns.

        The final size of the interpolated *data* field is determined by the 
        last indices in self.row_indices and self.col_indices.
        """
        row_interpol_data = self._interp_axis(data, 0)
        interpol_data = self._interp_axis(row_interpol_data, 1)

        return interpol_data