Laser image of the dune area of photo 2 with the dominant oblique dune direction 
filtered out (compare with the Fourier spectrum on photo 3b). The reality of 
a horizontal dune direction, previously unnoticed, is brought to light. 
Photo 
Laser image of the dune area of photo 2 with the horizontal dune 
direction filtered out (compare with the Fourier spectrum of photo 3c).