winds had died d.owi3.o Note that the visual, estimate of the significant height 

 was 7„ 5 feet at 1800 Z„ 



Stereo Contour Data 

 The first data prepared by the, Photogrammetry Division was iji the form 

 of a contour analysis of one of the stereo pairso It was soinewhat discomcert= 

 ing because the expected waves with lengths of from 100 to 300 feet or so 

 could not be seen in the contours and the range of contoured, heights was far in 

 excess of anything to be expected from a 20 knot wind„ 



The first hint of where the difficulty lay came from Woods Hole where lime 

 sections of the contoured surface were drawn. These sJkowed almost a, straight 

 line tilt along a given section with the waves we were looMngfor superi;rn.po!?ed 

 thereon-o A line section with arbitrary scale units from the lower left to the 

 upper right of the contoured surface is shown in figure 7.2o The un.foreseen 

 difficulty of determining a true mean zero reference plane on the open ocean 

 with no known reference points had arisen,, 



It was also pointed out at this time that spot heights could be deter= 

 mined by Hydro with far greater accuracy than the contauLi^s could be drawm 

 due to the nature of the techniques involved. The original plan had been to 

 choose an appropriate grid and read spot heights from the contoured, data. 

 This now had to be revised, and it was now necessary to find a way to deter- 

 mine the true zero reference plane and to choose a grid, the desired number 

 of points to be read, and the desired resolution and statistical reliability, 

 all on the basis of the data then on hand, 



51 



