Local attraction: It is the deviation or deflection of magnetic needle from its normal position under influence of external magnetic forces is called as local attraction.
Effect on prismatic compass: Due to local attraction, the bearings of the lines observed will be incorrect or having some error.
How is it taken care of:
1) The prismatic compass is set up for away from electric cable lines,L.T. line or transmission tower, electric and telephone poles.
2) Chains, steel tape, steel arrows, bunch of keys, steel wrist watches, steel buttons etc. are kept away from prismatic compass.
3) If local attraction still persists, it is corrected by mathematical calculation.