Simulated example of the spatial 2D correlation method. Panel a: simulation of a measured and shifted modal IM (x-slopes) for m^KL = 35. The grey and dark pixels correspond to the non-valid slopes w^valid(x) = 0 in the measured IM. Panel b: cross-correlation of the shifted mode of Panel a with the reference modal IM for m^KL = 35 (see Fig. 1). Panel c: map of the fitted coefficients α. Panel d: over-sampling of panel c with an interpolation via sinc functions.