134 CITY SURVEYING 



ANGULAR MEASUREMENT 



In city work, use is made of a transit having many features 

 that contribute to greater accuracy. The least reading of the 

 vernier is usually 30 or 20 sec., but sometimes angles are 

 required to a smaller unit than the least reading of the vernier. 

 These may be obtained by the method of repetition as follows: 

 The transit is set up over the vertex of the angle with the 

 verniers reading zero; the lower clamp being loosened and the 

 upper set, the telescope is directed along the left-hand side 

 of the angle. The lower clamp is then set, the upper loos- 

 ened, and the telescope directed along the right-hand side of 

 the angle. The upper clamp is now set, the vernier read, the 

 lower clamp loosened, and the telescope directed along the left- 

 hand side of the angle. The lower clamp is then set, the upper 

 loosened, and the telescope directed along the right-hand side 

 of the angle. The upper clamp is now set, the lower loosened, 

 and the telescope directed again along the left-hand side of 

 the angle; then the lower clamp is set, the upper loosened, and 

 the telescope directed along the right-hand side of the angle. 

 The process is repeated as often as necessary to obtain the 

 required accuracy. The vernier is read after the final .turning, 

 when the telescope is set on the right-hand side of the angle, 

 and the reading is divided by the number of turnings, including 

 the first. The result will be the value of the angle, which, as 

 a check, should closely approximate the first reading. This 

 first reading is taken only for the purpose of checking the final 

 result. 



Theoretically, the number of measurements should be such 

 that the sum will approximate a whole number of complete 

 revolutions, so that all parts of the circle may be used in 

 measuring; but, practically, three measurements are sufficient 

 in all ordinary cases. In very precise work, the angle may be 

 read as described, and then read again from right to left with 

 the telescope inverted. This eliminates errors of pointing 

 and adjustment of the line of collimation. 





