561 



5 . RESULTS 



The values of a number of parameters relevant to the 

 photogrammetric process are given in Table 1 . In 

 view of the preceding paragraphs this table is 

 largely self-explanatory but a few parameters will 

 be discussed briefly. 



The altitudes of photography are based on antic- 

 ipated significant wave heights and peak wave num- 

 bers. These were estimated by substituting the 

 windspeed and fetch in the JONSWAP parameter rela- 

 tionships [Hasselmann et al. (1973)]. For the sortie 

 off Sylt the wind information was fairly good as it 

 was based on ship observations in the area but for 

 the sorties off Noordwijk this information was poorer, 

 partly because no observations prior to the flights 

 were available. The helicopters were flying directly 

 into the wind during the sortie off Sylt. During 

 the second and third sortie they were flying with 

 the wind in the left respectively right rear quarter 

 with 11° drift. Ten stereo pairs were taken in each 

 sortie but one pair was rejected from the set taken 

 off Sylt because it covered too small an area. Us- 

 ing the sea surface information from the stereophoto- 

 grammetric analysis the three k-spectra were computed 

 according to the procedures described in Section 3 . 

 The results are presented in the form of countour- 

 line plots in Figures 10, 11, and 12. Some isolated 

 regions in the k-plane have been indicated where the 

 spectra are thought to be seriously affected by 

 noise. This noise is dealt with in the Appendix. 

 Values of relevant spectral parameters are given in 

 Table 2. For the deteirmination of the directional 



resolution, Og was estimated at 0.06 [c.f., 

 Holthiujsen et al. (1974)]. 



On closer inspection of the contour-line plot of 

 the spectrum of Sylt two wave fields can be identi- 

 fied: one coming from approximately 110° and one 

 from approximately 155°. This is rather surprising 

 because neither the wind conditions nor the ground- 

 true information gave such indication. The swell 

 in the second spectrum (off Noordwijk) coming from 

 south-westerly directions was observed during the 

 flight. It is well separated from the locally 

 generated wind sea and it will be largely ignored 

 in the following discussion. The peak of the third 

 spectrum is, surprisingly, coming from Northerly 

 directions rather than from Easterly directions, as 

 may be antitipated from the wind direction. 



Instead of the k, -spectra, the normalized direc- 

 tional distribution functions have been plotted in 

 Figures 13, 14 and 15. The definition of these 

 functions is given by Eqs. 12 and 13. 



D(e;k) 



D(e,-k) 



E(k,9) 



IT 



/E(k,9)de 

 



for < 



for IT < 



< 2-n 



(12) 



(13) 



This seemed to be more illustrative than a contour- 

 line plot of the k,9-spectra, the normalized direc- 

 primarily for the directional characteristics. An 

 evaluation of these functions will be given in the 

 next paragraph. 



