What is local attraction? How it is defected?
The compass needle doesn’t point to magnetic north or magnetic meridian when it is under the influence of the external attractive forced. It is found to deviate from its normal position when compass is placed in the vicinity of magnetic substances like iron and steel objects, iron ore, electric conductors carrying current. This deviation is called local attraction.
To detect the local attraction, the difference between fore bearing and back bearing of a line is found. If the difference is 1800 , then the end station of that line are not affected by local attraction and if it is not equal to 1800 , then either one or both end stations are affected by local attraction.