ANGULAR SURVEYING 



53 



and the a/imuth of CD is determined as explained for BC. 

 This azimuth is found to be 213 47 '. The instrument is 

 moved to D and oriented by backsighting on C. The forward 

 azimuth of CD being 213 47', the back azimuth is 213 

 47'+ 180 = 393 47', or 393 47' - 360 = 33 47'. After setting 

 the vernier at this reading and directing the telescope to C, 

 the transit is oriented at D. The lower clamp is then sec, the 

 upper clamp is loosened, the telescope directed to E, and the 

 vernier read again, the reading being the azimuth of DE. 



The magnetic bearing of DE is now taken; suppose it to be 

 N 77 15' E. As the declination is 2 26' west, the approxi- 

 mate true bearing of DE, as obtained from the compass, is 



N 74 49' E. Since the line has an azimuth of 74 34', its 

 true bearing is evidently N 74 34' E, which agrees with that 

 given by the compass within the limit of accuracy of the latter 

 instrument, with which angles are read to the nearest quarter 

 of a degree. In a traverse consisting of many lines, it is advis- 

 able to take the magnetic bearing of every third or fourth 

 line, and compare it with the true bearing obtained from the 

 azimuth of the line. 



