What is the Height of Instrument (HI) in leveling?

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

Answer: (A)

Height of Instrument is the vertical distance from the reference datum up to the instrument’s line of sight. This value represents how high the instrument is positioned above the datum and is the constant you use to convert rod readings into elevations. You determine it by taking a staff reading on a point of known elevation and adding that reading to the known elevation (HI = known elevation + staff reading). Then any point’s elevation is found by subtracting that point’s staff reading from HI (Elevation = HI − reading). This definition directly captures what HI measures—the height of the instrument’s line of sight above the datum—unlike rod readings or intermediate placements, which describe measurements or locations rather than the instrument’s height.