TOPOGRAPHIC SURVEYING 117 



CONTOURS 



Contour curves are lines joining points of equal elevations. 

 Fig. 5 illustrates part of a contour map of a survey. The 

 tract was divided by the lines AA', BB', CC', 1, 2, etc. 100 ft. 

 apart into squares of uniform size. Levels were taken at all 

 points of intersection and at any intermediate points where 

 the slope changes abruptly, and the positions of the con- 

 tour points were determined as follows: Take for instance 

 the line CC'. The elevation of Station C-0 is 39.1 ft., and that 

 of Station C-l, is 47.2 ft., giving a rise equal to 47.2 39.1 

 = 8.1 ft. from the former station to the latter. Since the 

 horizontal distance between the stations is 100 ft., the rate of 



slope is equal to or 12.3 ft. horizontal for 1 ft. rise. The 



8.1 



contour interval is taken at 5 ft., and, consequently, the eleva- 

 tion of each contour is some multiple of 5 ft. The first con- 

 tour above Station C-0 is contour 40, and to locate this 

 contour a rise of 40.0 39.1 = .9 ft. above this station must 

 be made. Since the rate of slope is 12.3 ft. horizontal for 1 ft. 

 rise, the horizontal distance from Station on this line to con- 

 tour 40 is equal to 12.3X.9 = 11.1 ft. The rise from contour 

 40 to contour 45 is 5 ft. As the rate of slope continues the 

 same, contour 45 will intersect line C at a distance of 12.3X5 

 = 61.5 ft. from contour 40, or 61.5+11.1 = 72.6 ft. from Sta- 

 tion 0. 



From an inspection of the elevation, it is evident that 

 contour 50 must occur between Stations 1 and 2, since the 

 elevation of the former is 47.2 ft., and that of the latter is 

 53 ft. The rise from Station 1 to Station 2 is equal to 53.0 

 47.2 = 5.8 ft. Since the horizontal distance giving this rise 



is 100 ft., the rate of slope is equal to = 17.2 ft. horizontal 



5.8 



for 1 ft. rise. To locate contour 50, a rise of 50.0-47.2 = 2.8 ft. 

 must be made, and since the rate of slope is 17.2 ft. horizontal 

 for 1 ft. rise, contour 50 will intersect line C at a distance of 

 17.2X2.8 = 48.2 ft. from Station 1. 



In a similar manner are determined the other points on the 



