TRAVERSE WORK. 



89 



conventions the character of the crossing, whether over, under, or grade 

 crossing. He should similarly describe all stream crossings, distinguishing 



fords, ferries, and bridges. 



MEASUREMENTS OF HEIGHT IN CONNECTION' WITH TRAVERSE LINES. 



Height measm-ements in connection Avith traverse lines are effected in 

 one of two ways — either by vertical angles with the telescopic alidade or 

 by the use of the aneroid. 



In regions where little or no secondary triangulation can be done, it 

 becomes necessary to accompany certain of the traverse lines by profiles 

 determined by vertical angles. Such profiles should be surveyed at inter- 

 vals of 4 or 5 miles where the contour interval is 20 feet, and at intervals 

 of 8 or 1 miles where it is 50 feet. 



The alidade generally used in running these profiles is of a small com- 



Fig. 10.— Small Telescopic Alidade. 



pact form, with low standards and short ruler. The telescope has low 

 power, but carries a good vertical arc and a level. The arc and vernier 

 are graduated to single feet with a radius of a mile, instead of degrees and 

 minutes, in order to facilitate computation. This graduation is made on the 

 assumption that where the angle is less than 5° the arc and the tangent do 

 not materially differ. 



With this instrument the plan of the traverse is run precisely as above 

 sketched, except that a rodman is frequently employed. In running the 

 profile, which is done coincident!}' with the plan, the points sighted for 

 elevation may be the same as are used for the plan. If a rodman is em- 

 ployed, the target on the rod should be set at the height of the instrument 

 to simplify record and computation. 



