docs: def _interp(self):
          """Interpolate the cartesian coordinates.
          """
          if np.all(self.hrow_indices == self.row_indices):
              return self._interp1d()
  
          xpoints, ypoints = np.meshgrid(self.hrow_indices,
                                         self.hcol_indices)
  
          for num, data in enumerate(self.tie_data):
              spl = RectBivariateSpline(self.row_indices,
                                        self.col_indices,
                                        data,
                                        s=0,
                                        kx=self.kx_,
                                        ky=self.ky_)
  
              new_data_ = spl.ev(xpoints.ravel(), ypoints.ravel())
              self.new_data[num] = new_data_.reshape(xpoints.shape).T.copy(order='C')