What is a backsight (BS) in differential leveling?

What is a backsight (BS) in differential leveling?

• A. Reading taken on a rod held on a point of known or assumed elevation.
• B. An intermediate point between two bench marks.
• C. Elevation of the line of sight of an instrument.
• D. Reading on a rod held on a point whose elevation is to be determined.

Answer: (A)

Backsight is the reading you take on the leveling rod when it’s held at a point with a known (or assumed) elevation. That reading, together with the known elevation, lets you determine the instrument’s height above the datum ( HI = known elevation + backsight ). With the height of the instrument established, you then read on points of unknown elevation (foresights) and compute their elevations as Elevation = HI − foresight. This back-reading step keeps the measurements tied to a known reference and provides the necessary reference height for all subsequent elevation determinations. The other options don’t fit because an intermediate point between benchmarks isn’t used to set the instrument’s height, the line-of-sight elevation describes the instrument height itself rather than a rod reading, and a rod reading on a point whose elevation is to be determined is a foresight, not a backsight.

Backsight is the reading you take on the leveling rod when it’s held at a point with a known (or assumed) elevation. That reading, together with the known elevation, lets you determine the instrument’s height above the datum ( HI = known elevation + backsight ). With the height of the instrument established, you then read on points of unknown elevation (foresights) and compute their elevations as Elevation = HI − foresight. This back-reading step keeps the measurements tied to a known reference and provides the necessary reference height for all subsequent elevation determinations. The other options don’t fit because an intermediate point between benchmarks isn’t used to set the instrument’s height, the line-of-sight elevation describes the instrument height itself rather than a rod reading, and a rod reading on a point whose elevation is to be determined is a foresight, not a backsight.