ANGULAR SURVEYING 33 



Precision. In chain surveying, an error of 1 in 500 is gen- 

 erally permissible, and should not be exceeded; that is, two 

 measurements of the same line should not give results dif- 

 fering by more than 1 ft. for every 500 ft. measured. If, how- 

 ever, the chaining is done carefully, and the ground is not 

 rough, the error need not exceed 1 in 800 or 1,000. 



ANGULAR SURVEYING 



COMPASS SURVEYING 



The compass used in surveying consists essentially of a mag- 

 netic needle supported freely on a pivot at the center of a hori- 

 zontal graduated circle. To this circle is attached a pair of 

 sights. The needle and graduated circle are enclosed in a brass 

 case having a glass cover, and the whole is attached to a tripod, 

 or Jacob's staff, by a ball-and-socket joint and is leveled by 



FIG. 1 



means of the plate levels. Fig. 1 illustrates the type of com- 

 pass in general use. 



Adjustments of the Compass. Besides several conditions 

 that are attended to by the instrument maker, the following 

 are indispensable for accurate work: 



