New Topographic Survey Methods 



437 



divided into links, with an additional half-chain "trailer." The 

 trailer is graduated in order to allow the proper slope measure- 

 ment for the excess of the hypotenuse of a triangle, the base of 

 which is 2 chains. For example, suppose the crew to be ascending 

 a slope as indicated in Fig. 2. The topographer reads from the 

 arc that the rise per chain on that slope is 38 feet. The rear 

 chainman (usually the timber estimator) then looks for the 38 

 etched beyond the 2-chain tag on the tape, which in this case 

 would be 20.42 feet beyond. By stretching the tape tight, the 

 point h is then located and is known to be 2 chains distant from 

 and 2 X 38 feet higher than a. The topographer then sketches his 

 contours, taking side shots in order to locate the distance between 

 contours, and "side walking" if necessary. 



Fig. 2. — ^Illustrating Use of Special Slope Tape 



A strip crew of two men will cover about two miles of line a day. 

 Frequently the brush and undergrowth are so thick that sights 2 

 chains long are impossible, requiring the use of a one-chain dis- 

 tance or sighting by sound. Remarkably good closures have been 

 obtained in many cases employing the latter method. In dense 

 and dark forest? it is usually necessary for the estimator to carry 

 a small mirror in order that the topographer may sight on the 

 flash. In this way sights are obtained through brush that seems 

 almost impenetrable to the eye. 



The general method followed is similar to that used in the aneroid 

 method. Primary controls fixing the geographic position of the 

 area both horizontally and vertically on the face of the earth are 

 obtained by precise connection to adjacent primary stations of the 



