Step Jf. — Measuring ai with protractor pinned at Pi, 

 Aa is then turned from the line P\Oi' by use of the degree 

 scale. "V\ hen a drafting machine is used, ai is measured 

 with an ordinary protractor. 



Step 5.— 

 protractor 



Aai at Pi Is determined from the graph on the 

 Since two contour intervals are being crossed, 



the- 



values for the two are added (i. e., 0.07). Aa is 



rigure 



turned at Pi to the right or so that ai is decreased. The 

 orthogonal is carried into P2, midway between ^3^3' and 

 didi . Two contour intervals cannot be crossed simulta- 

 neously from P2 shoreward as the refraction is too great. If 

 it were not so great to shoreward, P2, could be established 

 at the intersection of PiO/, and the contour didj'. 



16. 



25 



