PREPARING THE PLANTING AREA 83 



The reduced level is the height of the point levelled 

 above an imaginary level line called a datum-line, in this 

 case 10 feet below the starting-point. 



The line of coUimation is the height of the Une of 

 sight at each change of the instrument above the datum- 

 hne. 



The object in making the datum-line below the starting- 

 point is to avoid having minus figures. It may be made 

 so many feet above when working downhill. 



In the above booking 10-00 feet is taken as the reduced 

 level at A. The staff reading (backsight) at A (the level 

 of the water in the stream) is 7«31. This reading is added 

 to the reduced level, giving the height of collimation-line — 

 viz., 17-31. 



The next reading, 4-46, on the top of the bank a,t A, hi 

 subtracted from this, leaving 12-85 as the reduced level 

 for this point — i.e., 2-85 feet above the water-level. 



The instrument is turned round and a reading taken 

 from the staff at B (foresight) which is 4-71. This, too, is 

 subtracted from the height of the collimation, giving 12-60 

 as the reduced level at B. After setting up the instrument 

 midway between B and C, a reading (backsight) is taken 

 from staff at B. This is 5-50. 



To find the new coUimation-line this figure is added to 

 the last reduced level, viz., 12-60, making 18-10. The 

 staff is then moved to C, and a foresight reading made, 

 4«40. This is subtracted from the new height of collima- 

 tion, leaving 13-70 as the reduced level. 



The height therefore of any point above A may be read 

 direct from the reduced level coluron. 



