When is a backsight reading typically taken during leveling?

When is a backsight reading typically taken during leveling?

• A. After the instrument is set up and leveled.
• B. Before setting up the instrument.
• C. At turning points.
• D. Only when the rod is at unknown elevation.

Answer: (D)

Backsight readings are taken to establish the height of the instrument by comparing the line of sight to a point with a known elevation. After you set up the level and bring it to a level line, you first measure a backsight on a known point. This backsight establishes the height of the instrument (HI). With HI known, you then take foresight readings on points of unknown elevation to determine their elevations (Elevation of unknown = HI minus the foresight reading). So the typical timing is after the instrument is set up and leveled, using a known point for the backsight. The other options don’t fit because you don’t take a backsight before setup, it isn’t tied to turning points specifically, and it isn’t contingent on the rod being at unknown elevation.

Backsight readings are taken to establish the height of the instrument by comparing the line of sight to a point with a known elevation. After you set up the level and bring it to a level line, you first measure a backsight on a known point. This backsight establishes the height of the instrument (HI). With HI known, you then take foresight readings on points of unknown elevation to determine their elevations (Elevation of unknown = HI minus the foresight reading). So the typical timing is after the instrument is set up and leveled, using a known point for the backsight. The other options don’t fit because you don’t take a backsight before setup, it isn’t tied to turning points specifically, and it isn’t contingent on the rod being at unknown elevation.