How to detect local attraction ?

The magnetic needle is seriously deflected from its normal position when it is under the influence of external attractive forces. Such a disturbing influence is known as local attraction. The term is also used to amount of deviation of the needle form its normal position.

Detection of local attraction
The presence of local attraction at any station may be detected by observing the fore and back bearing of the line. If the difference between fore and back bearing is 180 degree, both end stations are free from the local attraction. If not, the discrepancy may be due to :

a) An error in observation of either fore or back bearing or both.
b) Presence of local attraction at either station.
c) Presence of local attraction at both the stations.

Correcting method

1) Method of elimination of local attraction by included angles

- Compute the included angle at each station from the observed bearings .
- Starting from the unaffected line, rub down the correct bearing of the successive sides.

2) Method of elimination of local attraction by applying correction in bearing

- Calculate the magnitude and direction of the error due to local attraction at each affected station.
- Run down the bearings, starting from the bearing unaffected by local attraction.