84 GARDEN DESIGN 



to reckon rise and fall which is done by finding the 

 difference between backsights and foresights or 

 between intermediates. An increase in reading 

 on the staff means a fall in the ground. Enter 

 the differences into their correct columns as rise 

 or fall. 



In the example we have the difference between 

 A and B, B showing a rise of 2-85. The next read- 

 ing gives a fall of 4-2 and the next -2and so on. The 

 final process is to find the reduced levels which 

 gives a progression of figures showing the relative 

 changes in the surface. As the reckoning is 

 from zero at A the first reduced level is the rise at 

 B 2-85. The fall from this gives 1-35 and the 

 rest of the column can be readily made out. 



These reckonings have to be proved both for 

 arithmetical mistakes and for errors of entering 

 rises into the fall column and vice versa. Add 

 up the backsights, and the foresights (not includ- 

 ing intermediates), and subtract. Add up rises 

 and falls and subtract. The results should both 

 equal the final reduced level. 



Working from zero over falling ground involves 

 minus quantities, which can be avoided by taking 

 a figure to work from. This is quite arbitrary, 

 and is usually 50 or 100, and is termed the datum. 

 With a datum of 50 the previous example would 

 appear as follows. 



