Reciprocal Levelling and the Error in Reciprocal Levelling due to Refraction

Reciprocal levelling is a technique used in surveying to eliminate errors caused by the curvature of the Earth and atmospheric refraction. It involves taking readings in both directions along a level line to cancel out any errors that may be present.

Non-adjustment of Line of Collimation
The line of collimation is an imaginary line that connects the crosshairs of the leveling instrument. It should be horizontal and parallel to the level line being measured. If the line of collimation is not properly adjusted, it can introduce errors in the leveling measurements. However, this error can be eliminated by using reciprocal leveling techniques.

Non-adjustment of the Bubble Tube
The bubble tube is a vital component of a leveling instrument that helps in ensuring the instrument is level. If the bubble tube is not properly adjusted, the instrument may not be accurately leveled, leading to errors in the measurements. However, this error can also be eliminated by using reciprocal leveling techniques.

Earth's Curvature
The Earth is not flat, and its curvature needs to be taken into account when performing leveling surveys over long distances. However, in reciprocal leveling, the error caused by the Earth's curvature is eliminated by taking readings in both forward and backward directions. This technique allows the curvature effect to be canceled out, ensuring more accurate results.

Refraction
Refraction is the bending of light as it passes through different media, such as the Earth's atmosphere. This bending can cause the apparent position of the target to be displaced, leading to errors in leveling measurements. In reciprocal leveling, readings are taken in both directions, which helps to cancel out the effects of refraction to some extent. However, refraction is not completely eliminated, and a residual error remains.

The error due to refraction occurs because the amount of refraction can vary depending on atmospheric conditions, such as temperature and pressure. These conditions can change from one direction to another, leading to differences in the refraction effects. Therefore, even though reciprocal leveling helps reduce the error caused by refraction, it cannot completely eliminate it.

In conclusion, the error that is not completely eliminated in reciprocal leveling is due to refraction. The bending of light as it passes through the Earth's atmosphere can cause displacements in the apparent position of the target, leading to errors in leveling measurements. While reciprocal leveling helps reduce this error, it cannot completely eliminate it due to variations in atmospheric conditions.