A magnetic needle indicate the north direction when freely suspended or pivoted. But if the needle comes near some magnetic substance such as iron ore, steel structure electric cable conveying connected etc.it is found to be deflected from its true direction and does not show the actual north this disturbing influence of magnetic substance is known as local attraction.

For example – consider the case when

Observed FB of AB = 60$^\circ$30’

Observed BB of AB = 240$^\circ$0’

Calculate BB of AB= 60$^\circ$30’+180$^\circ$0’= 240$^\circ$30’

Corrected BB of AB = 1/2(240$^\circ$0’+ 240$^\circ$30’)= 240$^\circ$15’

Corrected BB of AB = 240$^\circ$15’-180$^\circ$0’= 60$^\circ$15’