LEVELLING AND TOPOGRAPHY. 6$ 



elevation numbers show the actual fall from one point 

 to another. The engineer can now compute the area 

 of each drainage section. The topography is plain to 

 every one who may have occasion to examine the map. 

 It fits the ground so that most of the points can be 

 relocated from the map by their relation to natural ob- 

 jects and established features. An approximate esti- 

 mate of the cost of drainage can be made from this 

 map, though in order to arrive at an accurate estimate, 

 drain lines should all be measured. 



Topography by Contour Lines, 



Contour lines are drawn upon a map defining points 

 on the surface of the land which have the same eleva- 

 tion. The vertical distances between these lines may 

 be taken at 6 inches, i foot, or 2 feet, or any other de- 

 sirable distance, in which case the number of the con- 

 tour lines show to the eye at once the elevation of the 

 ground over which the line passes. The line of great- 

 est slope of the land will, of course, be directly across 

 the contour lines. 



For the purpose of illustration, suppose it is desired 

 to make a survey and map of a farm from which a plan 

 for its drainage is to be made and in time executed. 

 The earlier in the work such a map can be made, the 

 greater will be its value. Especially will such a map 

 be of service if the projected work is to be done at dif- 

 ferent times as facilities or means may permit. 



The Survey. — Begin at one corner of the farm or 

 field whose adjacent sides are straight lines and use 



