Reciprocal Leveling and its errors

Reciprocal leveling is a method of leveling in which the same set of leveling instruments is used to take readings in both directions along a given line. This is done to eliminate any systematic errors that may be present in the leveling instruments or in the observations themselves. However, some errors may still persist even after reciprocal leveling. One such error is refraction.

Refraction

Refraction is the bending of light as it passes through the atmosphere. This can cause the apparent position of a target to be different from its true position. Refraction occurs because the density of the atmosphere decreases with altitude, causing light to travel at different speeds at different heights. This bending of light can cause the line of sight to deviate from the horizontal plane, leading to errors in leveling observations.

Effects of Refraction

Refraction can have a significant effect on leveling observations, especially when the line of sight passes over long distances or over areas with large variations in temperature or atmospheric pressure. The effects of refraction are most pronounced in the early morning and late afternoon, when temperature inversions are most likely to occur.

Correction for Refraction

To correct for refraction, leveling instruments are equipped with a built-in correction factor that accounts for the effects of refraction. This correction factor is based on the average atmospheric conditions at the time and location of the leveling observations. However, this correction factor may not be accurate in all cases, especially when there are large variations in temperature or atmospheric pressure.

Conclusion

In reciprocal leveling, refraction is one of the errors that may still persist even after the method is used to eliminate systematic errors. Refraction occurs due to the bending of light as it passes through the atmosphere, and it can cause errors in leveling observations by deviating the line of sight from the horizontal plane. To correct for refraction, leveling instruments are equipped with a built-in correction factor, but this factor may not be accurate in all cases.