88 LEVELING 



line is heavy. By the aid of these heavy lines, distances and 

 elevations are quickly and correctly estimated and the work of 

 platting greatly facilitated. The elevations given in the pre- 

 ceding notes are platted in the accompanying diagram. The 

 elevation of some horizontal line is assumed. This elevation 

 is, of course, referred to the datum plane, and is the base from 

 which the other elevations are estimated. Every tenth station 

 number is written at the bottom of the sheet under the heavy 

 vertical lines. 



Grade Lines. The principal use of a profile is to enable the 

 engineer to establish a grade line; that is, a line showing the 

 slope of the road on which the amounts of excavation and 

 embankment depend. The rate of a grade line is measured 

 by the vertical rise or fall in each hundred feet of its length, 

 and is designated by the term per cent., abbreviated %. Thus, 

 a grade line that rises or falls 1 ft. in each hundred feet of its 

 length is called an ascending or a descending 1 % grade, and is 

 written + 1 or 1 per hundred. A rise or fall of $ ft. in each 

 hundred feet is called a .5% grade, and is written + .5 or .5 

 per hundred. The grade line having been decided on, it 

 is drawn in red ink, and the rate of grade is written on 

 the line. 



EXAMPLE. The elevation of station 20 is 140 ft.; between 

 stations 20 and 100 there is an ascending grade of .75%. What 

 is the elevation of the grade at station 71? 



SOLUTION. To obtain the elevation of the grade at sta- 

 tion 71, add to the elevation of the grade at station 20, or 

 140 ft., the total rise in grade between stations 20 and 

 71. The distance is 71-20 = 51 stations. The total rise is, 

 therefore, .75ft. X 51 = 38.25 ft.; 140 ft. +38.25 ft. -178.25 ft., 

 the elevation of grade at station 71. 



ACCURACY IN LEVELING 



Curvature and Refraction. Owing to the spherical form of 

 the earth, the difference in elevation, as shown by the rod read- 

 ing, between the line of sight and the point on which the rod is 

 held is not equal to the difference in elevation between the 

 cross- hairs and the point, the rod reading being in excess of the 

 true difference in elevation. 



