LEVELLING AND TOPOGRAPHY". 59 



Proof : Difference of elevation between A and 

 D = 14.9s — 10.00 = 4.95. 



Sum of back sights = 14.20 

 Sum of foresights = 9.25 



Difference of elevation = 4.95 



Drainage Topography, 



Drainage surveys involve the collection and repre- 

 sentation of such facts relating to the surface of the 

 land, its soil and subsoil, as will be of service in de- 

 termining upon and carrying out a plan for the profit- 

 able drainage of the land so examined. It is a branch 

 of work that comes within the special province of the 

 drainage engineer, and it must be done with greater 

 or less thoroughness before he can plan a drainage pro- 

 ject with any assurance that it will accomplish the de- 

 sired work when carried out. 



The completeness of a drainage survey must be meas- 

 ured by the time and money that can be devoted to 

 it, and by the thoroughness of the drainage work for 

 which the survey is made. There should be an adop- 

 tion of these factors to the result aimed at. The en- 

 gineer should understand what data and information are 

 required, and then make his plans to obtain them in 

 the most systematic and expeditious manner possible, 

 otherwise he will fritter away his time and energy upon 

 matters that do not pertain to the case in hand. A 

 survey may be required of a field, farm, district, town- 

 ship, or county, yet in all cases the work must be con- 

 ducted so as to cover the points that will be required 



