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 taken on a rod held at a point with known or assumed elevation. This reading ties the instrument to a reference datum, establishing the height of the instrument (HI) by adding the backsight to the known elevation. Once HI is set, you move to other points and take foresight readings to determine those elevations, using the relationship Elevation = HI − foresight. The other described scenarios don’t fit: a point between benchmarks isn’t a backsight, the line of sight elevation isn’t the backsight reading, and a reading on a rod at a point whose elevation is to be determined is a foresight.