Ensure Verticality of Plane of Sight:

A plumb bob is suspended in front of the compass set in position and is observed through the instrument. The eye vane, the object vane and the string of plumb bob shall be in same vertical plane. Any deviation will indicate the loss of verticality of either the eye vane or object vane which is then adjusted accordingly.

Closing Error

When a closed traverse survey is conducted and the results plotted, it may be observed that traverse fails to close. Actual distance by which traverse fail to close is called the closing error. These could be either because of

(a) Error in measuring angles, or

(b) Error in measuring distances.

All the included angles of the traverse are computed from the recorded bearings and aggregated. If the aggregated involved angle is equal to (2n - 4) right angles, angle measurements are correct provided there is no local attraction influence or observational error. Any difference will indicate error in angular measurement. If the closing error is large, the survey is rejected and repeated. If it is little, the error could be corrected through making small adjustments in bearings as explained further "Adjustments and Corrections".