A METHOD OF ESTIMATING WAVE DIRECTION 



Do Ro B'orrestj Project Engineer 

 Bsach Erosion Board 



For the past several months a field research group of the Beach 

 Erosion BDard has been engaged in making hydrographic surveys and 

 daily observations at Mission &y, San Diego, California » The daily 

 observations consist of estimating wave period^ height and direction, 

 local wind direction and velocity, and littoral drift direction and 

 velocity,, 



Of the several observations, a satisfactory method of estimating 

 wave direction has presented the greatest problem. This has been over- 

 come to some extent by using a sighting bar (figure 1) and auxiliary 

 sights attached to an ordinary engineer's transit (figure 2)o An 

 automatic recording device would be superior but lacking such a device 

 the sighting bar has yielded generally acceptable results o 



The sighting bar consists of a 3/l6-inch diameter brass rod 8 

 inches long, one end of which is threaded to fit in the place of the 

 end cap of the transit's horizontal spindle. Several such rods have 

 been procured from a local machine shop at a nominal cost. In order 

 that the rod accurately extends the horizontal axis of the transit, 

 the rod must be straight and the threads cut to a square shoulder. 



The rod is attached to the transit only at the time of use but 

 the small aluminum auxiliary sights, to aid in pointing the tele- 

 scope, have been permanently installed, Ihese sights are held in 

 place by the capstan adjusti.ng nuts of the level tube, with line of 

 sight made parallel to the optical axis of the transit telescope. 

 They are inconspicuous and do not interfere Tvith any other use of 

 the transit. 



To measure wave direction, the transit is set up over a point 

 of knowi location and elevation and oriented on a distant point of 

 known direction. Since the sighting bar, when attached, revolves in 

 a horizontal plane approximately parallel to the sea surface, it is 

 possible to align the bar visually with a wave crest and read the 

 orthogonal azimuth on the plate of the transit directly. When the 

 sighting bar is in alignment or parallel with the sighted wave crest, 

 the telescope will be oriented perpendicular to the wave crest extend- 

 ed or in the direction of the orthofpnal. The telescope will not be 

 pointing toward the position of the sighted wave, unless the particular 

 wave cnosen is traveling directly toward the observer. An observer 

 may obtain best results in aligning the bar to the wave crest, by 

 sighting with one eye closed (figures 3 and 4.), To obtain the 

 position of the wave which was sighted, the transit is simply pointed 



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