ELEVATIONS. 77 



duced much oftener. At each azimuth station the new astronomic azimuth 

 should be adopted in place of that carried forward, and in case the discrep- 

 ancy between the two is sufficiently great to involve perceptible error upon 

 the scale of the map, the correction should be uniformly distributed forward 

 from the first station. 



In running these traverses all road crossings should be located, as 

 topographic traverses will be run over the roads and will be connected with 

 the primary traverses at these points. All prominent houses or natural 

 features of any kind in sight from the line must be located by intersection, 

 as they will doubtless be used by the topographers for location. 



When traversing in a country which has been surveyed by the General 

 Land Office into townships and sections, the crossing of every township 

 and section line should be located, and the directions of the township lines 

 with reference to the line of traverse should be carefully measured in order 

 to establish as close a relation as possible between the traverse line which 

 serves as ultimate control, and the township system of surveys, which serves 

 as a secondary control. 



Lines of traverse exceeding 100 miles in length should be reduced by 

 computation. The distances should be corrected for error of tape, for tem- 

 perature, and slope, and should be reduced to sea level, in the same man- 

 ner as above described in treating of the reduction of base lines, in case 

 these corrections are of sufficient amount to affect the length appreciably 

 upon the map. 



The courses should be corrected for convergence of meridians. Then, 

 commencing at the initial point, the latitude and departure of each station, 

 one from another, should be computed in feet. The sum of the latitudes 

 converted into seconds of latitude gives the difference in latitude, and the 

 sum of the departures converted into seconds of longitude gives the differ- 

 ence in longitude. 



Short lines of traverse maybe platted with minute reading protractors, 

 but in this platting the utmost care should be exercised. 



PRIMARY ELEVATIONS. 



The initial elevations of this work are derived from various sources. 

 Any trustworthy results known to be of a sufficient degree of accuracy for 



