Errors in levelling | Civil Easy Learning

Errors in levelling:

Collimation, Parallax

Change point / staff instability

Instrument or Benchmark instability

Refraction

Uncalibrated staff or levels

Reading, booking, or computation errors

Fore- and backsight distances different

Systematic and Random Leveling Errors:

There are a large number of potential sources of errors in leveling. Many of these are only significant for precise leveling over long distances. For the short segments of leveling that will occur in connecting a TGBM to nearby benchmarks there are only four worth mentioning: 

·        Collimation Error

·        Error due to Earth Curvature

·        Error due to Parallax Error

·        Error due to Refraction

Collimation Error:

The Automatic Prism compensator goes out of alignment.

The level provides readings outside of its specification

Error due to Curvature of the Earth:

Due to the curvature of the Earth, the line of sight at the instrument will deviate from a horizontal line as one moves away from the level

Correction of Curvature Error:

For a sight length of 300 ft the effect is only 1mm.

Keep Sight lengths under 150 ft

 the effect is eliminated by using equal sight lengths for fore- and backsights. 

Error due to Parallax Error:

When using an optical instrument — both the image and cross hairs can be focused- if either is imprecisely focused, the cross hairs will appear to move with respect to the object focused, if one moves one's head horizontally in front of the eyepiece.

Error due to Refraction: