orthogonal. The figure is exaggerated to be sure, but since the 

 determination of an angle of turning depends on the ^accurate 1 * 

 determination of the direction of a level line, it can be seen that 

 a seaward projected orthogonal need not coincide with one projected 

 shoreward over the same underwater topography. Since this is so, 

 it would seem that the simpler' method of constructing a seaward 

 orthogonal is to be preferred. 



Deep wafer 



Contour 

 £eire/ .//jgg. 



Conrour 



leveJJiSS-— 



S6o>//o cJ u>£> ■/■£ r 



It might be pointed out that the means prescribed in H. 0. Publ. 

 No. 605 for the determination of a level line is the one major weakness 

 of the orthogonal method as presented in that paper. Assumption 5 on 

 page 19 requires that a level line (make) w .... equal angles with the 

 adjacent contours ..."% that is, that its direction is to be determined 

 by a normal to a line representing the gradient between contours. An 

 orthogonal promoted rarely coincides with this direction of shortest 

 distance between contours. 



Figure 1 is a drawing comparing the accuracy of the two methods 

 of seaward orthogonal projection with a standard shoreward projection. 

 It was constructed by first drawing a standard orthogonal, and with 

 the shallow water position and direction so determined constructing 

 the two seaward projected orthogonals. The total angle of turning is 

 indicated by the azimuths given at the seaward and shoreward ends of 

 the orthogonals. 



■R- #■ -K- K * 



Author's Comments ; 



It is interesting to note the similarity in the two methods of 

 solution to the problem of drawing orthogonals seaward from shore. 

 The use of the new protractor represents a logical extension of Mr. 

 Lowe's method, and since it substitutes a mechanical determination of 

 the angle change for a mathematical computation, would seem to be 

 preferable. Both methods, as indicated in Mr. Kaplan's figure, give 

 almost identical results. 



20 



