DIFFERENTIAL LEVELING

• BENCH MARK -A fixed point of reference whose elevation is either known or assumed.
• BACKSIGHT -a rod reading taken on a point of known elevation (BM or TP) - usually taken with the level sighting back along the line - sometimes called a plus sight
• BACKSIGHT DISTANCE -horizontal distance from the level to rod on a backsight
• FORESIGHT -a rod reading taken on a point of unknown elevation (BM or TP to be established) • sometimes called a minus sight
• FORESIGHT DISTANCE -horizontal distance from the rod to level on a foresight
• TURNING POINT -an intervening point between two bench marks upon which point foresight and

  backsight rod readings are taken
• HEIGHT OF INSTRUMENT -elevation of the line of sight of the telescope above the datum when the instrument is leveled
• DIFFERENTIAL LEVELING -it is the process of determining the difference in elevation between two or more points some distance apart.
• Equipment used in differential leveling:

  • Graduated Leveling Rod

  • Dumpy Level
• THREE-WIRE LEVELING - A more precise method of differential leveling can be attained by this method. Three horizontal hairs are read and recorded instead from a single horizontal hair.
• DOUBLE-RODDED DIFFERENTIAL LEVELING -It is a method of determining difference in elevation between points by employing two level routes simultaneously.