What is typically required for successful three-wire leveling?

In three-wire leveling, the fundamental principle revolves around the accurate measurement of differences in elevation using three reference points known as stadia hairs. The three-wire method involves taking readings from both the upper and lower stadia hairs of a level rod, along with the central hair. This allows for compensation of errors that may occur due to instrument tilting or slight mispositioning when aiming at the leveling rod.

Averaging the three readings helps to ensure a more precise calculation of the height, as it mitigates any potential inaccuracies caused by changes in the line of sight or minor fluctuations in the rod's position. The result is a more reliable measurement for determining elevation differences over long distances or uneven terrain.

The other choices do not align with the principles of three-wire leveling. For instance, relying on only one rod measurement would not provide enough data for accurate leveling, and taking two measurements at different points would not utilize the advantages of averaging multiple readings from the same rod. Lastly, while a laser range-finder can provide accurate distance measurements, it does not specifically relate to the traditional technique of three-wire leveling, which fundamentally relies on optical instruments and manual readings for elevation determinations.