PREPARATION OF THE DESIGN 85 



The reduced level 42-63 is 7-37 below datum 50. 



The datum of a survey i.e. its starting point 

 should be at a place easy to locate, as all future 

 levels have to be referred to it for comparison. 

 Strong square pegs are driven at the places where 

 the staff has been set up to be read, and numbered, 

 and on the plan the numbers and the reduced levels 

 for each peg are entered. It is convenient in pre- 

 paring a ground to set it out in squares for then 

 the same levels enable sections to be plotted from 

 north to south as well as from east to west. 



Proving the figures in the level book is no 

 guarantee that the levelling itself has been ac- 

 curately done : figures on the staff may have been 

 misread, or the instrument may not have been truly 

 set up. It is therefore necessary to check the 

 figures by levelling back over the ground to the 



