a) Digital holographic image of GUW on the surface of an aluminum sample of 32.0 cm × 9.4 cm × 2.6 cm resulting from a piezo excitation with f_Exc = 200 kHz. b) By statistical analysis of the measured height distribution with the SF, the wavelength of the GUW is given by the position of the first local minimum at s₀ = 10.7 ± 0.8 mm. Overall, the multidirectional SF makes a periodicity visible, despite the superposition of various waves on the surface.