94 STADIA AND PLANE-TABLE SURVEYING 



STADIA AND PLANE-TABLE 

 SURVEYING 



STADIA SURVEYING 



Stadia surveying is the process of determining distances by 

 observing through a telescope (usually that of a plane table 

 or a transit) the intercept on a graduated rod. The intercept 

 is formed by two horizontal cross-hairs, which are 'carried on 

 the same reticle as the regular cross-hair and are equidis- 

 tant from it. The intercept bears a certain relation to the 

 distance of the instrument from the rod. The instrument 

 is also provided with a vertical circle, so that the vertical angle 

 that an inclined sight makes with a level line may be measured. 

 This angle serves for determining horizontal distances, as well 

 as for figuring the relative elevation between the instrument 

 point and the point where the rod is held. When the line ot 

 sight is nearly level, the distance d of the instrument from the 

 rod can be determined by the formula: 

 rf-xR-H, 



in which R denotes the stadia reading or the intercept between 

 the stadia wires, and s and are called, respectively, the stadia 

 constant and the instrument constant. Their values are usu- 

 ally determined by the instrument maker. The instrument 

 constant varies from about .75 to 1.33 ft. in different tran- 

 sits, according to the size and power of their telescopes. Its 

 value is usually marked on a card attached to the inside of 

 the instrument box. 



The stadia constant is customarily made equal to 100; so 

 that, in a horizontal line of sight, the stadia wire will intercept 

 a distance of 1 ft. on a rod whose distance from the instrument 

 is 100 ft. plus the instrument constant. Thus, if the stadia 

 wires intercept a distance of 8.37 ft. on the rod, the distance 

 from the rod to the transit would be 837 ft. plus the instrument 

 constant. For ordinary topographical work, especially for long 

 distances, it is sufficiently close to take for the distance 

 100 times the length intercepted on the rod, the instrument 



