DRAINAGE 



Figure 58 and notes. For some purposes the long 

 mason's level may be used, and levels may even be 

 determined by setting stakes above the water level in a 

 pond or lake. By having the tops of these stakes all 

 extend the same distance above the water, a level line 

 may be projected by sighting across their tops. 



Use of the datum plane. In comparing the height of 

 the different points along the main drain, and also along 



Fig. 81. Blank form used in recording notes of levels taken while planning for 

 a drain. 



its branches, some method is necessary by which 

 the relative heights of all these points may be ascer- 

 tained. There are several ways of doing this, 

 but what is wanted is a simple plan of calculation 

 which will be accurate and will clearly show the 

 relative height of points with each other and es- 

 pecially with the outlet. The following plan is in 

 common use, and with practice can be employed 



