

Fig. 60 



spacing the stadia wires gives uniformly reliable results. They 

 are equally spaced above and below the central wire so that distan- 

 ces between a X an d /^ mile can be estimated with the ordinary 

 leveling rod, by the half interval of 1:200. 



The constant (c + f), or the distance from the center of the 

 instrument to the anterior focal point, was a slightly variable factor 

 due to the process of focusing with the older style of draw-tube, 

 until it was made uniform by our cryptic focus. It is applied 

 to all observations of whatever size, irrespective of whether the 

 whole or half wire interval is used. It may be determined, by 

 focusing the telescope on some distant point, then by measuring 

 the distances from the center of the objective back to the diaphragm, 

 adding to this the distance from the objective back to the center of 

 the telescope axis. 



The true formula for calculating all horizontal distances by the 

 stadia method is :- D = KI -f- (c + f) but the last fa< : 

 sometimes ignored on long sights if the allowable error will permit 

 it. For certain work we occasionally fix the Interval, by 



request, so that it will read exactly 5 :500. If it reads 5 ft. on a 

 rod held 500 ft. from the center of the instrument, the interval is 

 obviously larger that it should be. All distances read by such an 

 interval, except the one for which the computation was made, 

 will be slightly in error. 



The specifications of the General Land Office call for a fixed 

 stadia interval of 1 : 132. We quote from the text in the Depart- 

 ment Field Manual: 



"In public land surveying it is convenient to have fixed stadia 

 wires with a ratio of 1:132 so that the sum of two rod readings in 

 feet will be equivalent to a ratio of 1 : (>6, or a reduced distance in 

 chains; it is also convenient to reduce the error in the wire intenal 

 to the error in 10 chs., and to eliminate the error by applying to 



