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 transfer a known elevation to the instrument. After you set up and level the telescope, you take a backsight on a rod held at a point whose elevation is known to establish the height of the instrument (HI). Once HI is known, you can read foresights on points with unknown elevations and compute their elevations from HI and the foresight readings. Using a backsight only when the rod is at an unknown elevation provides no reference to anchor the measurement, so elevations couldn’t be determined reliably.