Local attraction
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While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called as local attraction.[1] The external forces are produced by sources of local attraction which may be current carrying wire (magnetic materials) or metal objects.[2] The term is also used to denote amount of deviation of the needle from its normal position. It mostly causes errors in observations while surveying and thus suitable methods are employed to negate these errors.[3]
The sources of local attraction may be natural or artificial. Natural sources include iron ores or magnetic rocks while as artificial sources consist of steel structures, iron pipes, current carrying conductors. The iron made surveying instruments such as metric chains, ranging rods and arrows should also be kept at a safe distance apart from compass.[3]