at it, using the auxiliary sights , and the azimuth and vertical angle 

 reado Sighting through the telescope is unsatisfactory since the 

 field of vievr is too small and the magnification undesirable,, 



The higher the observation station the better, provided it is not 

 too distant from the sea area where the wave direction is to be 

 measured. At Mission ;^y two stations are in useo The one most used 

 is UoSoCo and GoSo triangulation station "Jolla" which provides a 

 height of instrument of elevation 535 06 msl and is located approximately 

 4,000 feet from the beach » A chart (figure 5) has been plotted showing 

 the relation of vertical angle to distance from the observation stations, 

 Ihe maximum distance which can be determined with reasonable accuracy 

 is about 20,000 feet for station "Jolla", The chart has been corrected 

 for the earth's curvature and refractiono The maximum distance from an 

 observation station that swell direction can be m.easured with the 

 sighting bar seems to be that obtained try a vertical angle of about l|- , 

 For smaller angles the errors in locating a wave and measuring its 

 direction become excessive, Ihe larger the vertical angle the more 

 accurate is the position and direction of the wave determinedo However, 

 it appears desirable to make observations as far from shore as practica- 

 able and a compromise is thus required, Tfith some sacrifice in accuracy, 

 With the azimuth and distance to the observed point of the wave crest 

 thus knovm, the point is plotted and the orthogonal azimuth drawn. 

 This may or may not be the deep-water direction, depending upon the 

 wave period. At Mission Biy, for periods of 10 seconds or more, com.- 

 parison with wave refraction diagrams is necessary in order to obtain 

 deep-water direction, 



A test of the accuracy of the method was nsde by making observa- 

 tions at the same time aerial photos were being flown o A controlled 

 mosaic (figure 6) was made and the observed wave directions plotted 

 thereon. Though the photography left much to be desired, several 

 satisfactory comparisons were possible „ It the time of observation 

 three separate wave patterns existed. One had a very short period and 

 was not identifiable from shore. The other tvro had estimated wave 

 heights of about 2 feet and had relatively long periods. Of 10 shore 

 observations which plotted in areas on the photos in which swell could 

 be identified, the average error in wave direction appeared to be about 

 3° with a maximum error of 5°o 



There are several features which limit the usefulness of the sight- 

 ing bar in measuring the direction of vrave fronts, namely: 



1, It is nonrecording and requires an observer, 



2. Observation can be made only during daylight and when 

 fog, rain, or haze conditions are not prohibitive, 



3o A high observation station close to the sea is 

 desirable. 



34 



