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G. Three-Wire Leveling

1. Principle

Three-wire leveling is so named because it uses all three horizontal crosshairs (wires) for backsight (BS) and foresight (FS) readings. The top and bottom are commonly called stadia crosshairs or stadia wires, Figure G-1, and are equally spaced on both sides of the center wire.

Figure G-1 Stadia Wires

Reading all three wires:

• allows distance determination (stadia)
• refines and checks BS and FS readings

The stadia principle is described in V. Distance Measurement Methods.