ANGULAR SURVEYING 



35 



To Center the Needle Pivot. Having, if necessary, straight- 

 ened the needle, turn the compass so that the north end of the 

 needle will exactly cut some prominent graduation mark, and 

 observe whether the south end exactly cuts the opposite grad- 

 uation mark. If it does not, find the position of the needle 

 that shows the greatest difference in the readings of its opposite 

 ends; then remove the needle from the pivot and bend the 

 pivot carefully at right angles to this position an amount equal 

 to one-half the error. Repeat the operation until the needle 

 cuts accurately all opposite graduation marks. 



Use of the Compass. By means of the compass the angle 

 between any line and the direction of the needle, or the mag- 

 netic meridian, can be measured directly. This angle is called 

 the magnetic bearing of the line. The angle between two lines 

 can be determined by either subtracting or adding their bear- 

 ings, as the case may require. A rough sketch, showing the 

 relative positions of the two lines with reference to the meridian, 

 will enable one to determine by inspection the required arith- 

 metical operation. 



Bearings are reckoned from 

 to 90 and indicate the amount 

 by which a line is east or west 

 of north or south. In Fig. 2, 

 in which NS represents the 

 magnetic needle, N and 5 being, 

 respectively, the north and 

 south ends, the line OPi makes, 

 with the north half of the needle, 

 an angle of 60. As the line 

 lies between the north point N and the east point E, its bear- 

 ing is 60 northeast or 60 to the east of north. This is indi- 

 cated by the notation N 60 E. Similarly, the bearings of 

 OPt, OP 3 , and OPi are, respectively, N 42 W, S 70 W, and 

 S 50 E. 



To determine the magnetic bearing of a line, turn the com- 

 pass, after it has been set and leveled, until the line SN, Fig. 3, 

 which is the line of the sights, coincides with the line OP whose 

 bearing is to be determined, the observer's eye being at the 

 slit near S. The north end of the needle FG is then pointing to 



FIG. 



