LEVELING 85 



point will be equal to the H. I. minus the foresight 

 reading. 



A turning point is a point where the rod is held for a fore- 

 sight, and after the level has been moved to a new position, for 

 a backsight. The backsights are (+) readings, asid are to be 

 added; the foresights are ( ) readings, and are to be sub- 

 tracted. A point for a foresight having been determined, the 

 rodman drives a peg firmly in the ground and holds the rod 

 upon it. After the instrument is moved, set up, and a back- 

 sight taken, the peg is pulled up and carried in the pocket 

 until another turning point is called for. 



Balancing Backsights and Foresights. The most valuable 

 and reliable safeguard against errors in leveling is obtained by 

 equal backsights and foresights on turning points. They should 

 usually be equal in pairs; that is, each pair of sights on turning 

 points, one backsight and one foresight, should be of approxi- 

 mately equal lengths. Should any inequality of length occur 

 in one pair of sights, it should be balanced up in the next pair, 

 or as soon as possible. For example, should the foresight in one 

 pair of sights be longer than the backsight, then in the next 

 pair of sights the backsight should be made correspondingly 

 longer than the foresight. The sights should be balanced as 

 perfectly as possible between bench marks. It is not neces- 

 sary to measure the lengths of the sights accurately; they 

 can be determined closely enough by counting steps in walking. 

 A man of ordinary stature, when walking naturally, will average 

 about 40 steps in each 100 ft. of distance, usually a somewhat 

 less number on smooth and level ground, and a greater number 

 where the ground is rough or sloping, either ascending or 

 descending. 



Keeping Level Notes. Many forms on which to keep level 

 notes are used. The distinguishing feature of one of the best, 

 which is here shown, is a single column for all rod readings. 

 The backsights being additive and the foresights subtractive 

 readings, they are distinguished from other rod readings by the 

 signs + and . 



Checking Level Notes. A well-known method of checking 

 level notes provides for checking the elevations of turning 

 points and heights of instrument only, which is sufficient, as 



