569 



Waterway, Port, Coastal and Ocean Division, WW2 , 



paper 12924, p. 251. 

 Stilwell, D. (1969). Directional Energy Spectra of 



the Sea from Photographs. Journal of Geophysical 



Research, 74, 8; 1974. 

 Sugimori, Y. (1975) . A study of the application of 



the holographic method to the determination of 



the directional spectrum of ocean waves. Deep- 

 Sea Research, 22, 339. 

 Tyler, G. L., C. C. Teague, R. H. Stewart, A. M. 



Peterson, W. H. Munk, and J. W. Joy (1974) . Wave 

 directional spectra from synthetic aperture ob- 

 servations of radio scatter. Deep-Sea Research, 

 21, 989. 



APPENDIX 



NOISE 



Inspection of contour-line maps of the sea surface 

 obtained from the observation off Sylt revealed a 

 dome-shaped distortion. This distortion is probably 

 caused by the fact that the pictures could not be 

 positioned in the stereoscopic viewing devices with 

 the accuracy normally obtained with high grade pic- 

 tures. When this positioning is not optimal, a 

 dome-shaped distortion is to be expected. Unfor- 

 tunately the exact distortion cannot be determined, 

 but in the k-plane it seems to be well separated 

 from the wave information (area No. 1 in Figure 19) 

 and the data in this area was removed in the sub- 

 sequent analysis. 



The other noise-affected areas are related to a 

 phenomenon introduced by the manner of scanning 

 the pictures during the photogrammetrical process : 

 the sea surface elevation at even-numbered lines 



(scanned in positive y-direction) is systematically 

 slightly too low, while the elevation at odd- 

 numbered lines (scanned in negative y-direction) 

 are systematically slightly too high. This effect 

 has been observed earlier in the analysis of stereo 

 photos of regular waves generated in a hydraulic 

 laboratory. The principal wave length and direction 

 of this distortion correspond with the location of 

 area No. 2 in Figure 19, which is the location of 

 the Nyquist wavenumber in x-direction. This spectral 

 information was removed from the spectra in the sub- 

 sequent analysis. The noise in areas No. 3, 4, and 

 5 was labeled as such mainly because of the delta- 

 type behavior of the directional distribution func- 

 tions in these regions. It is probably due to 

 variations in the error introduced by the scanning 

 and possibly also by "leakage" from area No. 2. In 

 the k-spectrum off Sylt this noise was not removed. 

 In the k-spectrum off Noordwijk the noise in the 

 indicated region in Figure 11 has been removed. 



/y^r^ I i 





0.01 0.02 0.03 aD4 0.05 0,06 007 0.08 009 0.10 

 wavenumber component k I m" j 



FIGURE 19. Location of noise in 



->- 



k-plane . 



