The Abney Hand Level. 353 



The work in vertical location along the strip is' valuable only 

 to guide the office man in the positioning of the contours upon the 

 form line field sketch turned in by the compassman. Consequently 

 if the field man preserves the general profile of the strip the 

 elevations secured along his line will be accurate enough for all 

 practical purposes of the work. The precise elevations of particu- 

 lar points along the line are of no moment. The relative value 

 of the slopes the one to the other along the line are of intense 

 practical value to the man in the office when plotting the final 

 map. Errors in absolute elevation along the strip line are con- 

 trolled by the precise elevations of the base line stakes at each 

 end of the strip line. 



In order to handle the Abney intelligently the compassman must 

 clearly understand the theory of the Abney level and its practical 

 application in obtaining elevations along the line on a strip sur- 

 vey. The geometric principle is readily understood; the slope 

 is measured either in degrees or per cents of slope ; the tangent 

 of the vertical angle represents the per cent of slope ; the two are 

 synonymous ; in descending a slope of two per cent the com- 

 passman drops two feet in elevation in every 100 feet of hori- 

 zontal distance. 



The real difficulties in the use of the Abney are encountered 

 in its practical application. The compassman is too apt to be 

 confused by every little irregularity of the ground. Before at- 

 tempting work with the Abney the compassman must under- 

 stand that the object of his work is to obtain a general profile 

 of the ground along the strip. The final map is drawn on a 

 scale of four inches to the mile. The small irregularities which 

 attract the attention of the compassman are lost sight of in the 

 preparation of the final map. The accurate approximation of the 

 per cents of the slopes along the strip becomes the guide of the 

 office draftsman when he expresses the character of the various 

 felopes by contour lines. Then the value of the slope data be- 

 comes most apparent and the desire for the representation of the 

 minor points of relief is recognized as impractical and unneces- 

 sary. 



The compass man need not sight upon any definite object. His 

 sight must be largely influenced by judgment and this judgment 

 can be practiced so that the compass man will be able to approxi- 



