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''. The external forces are produced by sources of local attraction which may be current carrying wire (magnetic materials) or

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...

objects. 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 Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ...

and thus suitable methods are employed to neglect these errors."Compass Surveying" by C. L. Kocher, Principal of Mehar Chand Polytechnic College,

Jalandhar Jalandhar is the third most-populous city in the Indian state of Punjab and the largest city in Doaba region. Jalandhar lies alongside the Grand Trunk Road and is a well-connected rail and road junction. Jalandhar is northwest of the state ...

City,

Punjab Punjab (; Punjabi: پنجاب ; ਪੰਜਾਬ ; ; also romanised as ''Panjāb'' or ''Panj-Āb'') is a geopolitical, cultural, and historical region in South Asia, specifically in the northern part of the Indian subcontinent, comprising ...

Sources

The sources of local attraction may be natural or artificial. Natural sources include

iron ores Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...

or magnetic rocks while as artificial sources consist of steel structures, iron pipes, current carrying conductors. The

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

made

instruments such as metric chains, ranging rods and arrows should also be kept at a safe distance apart from

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

Detection

Local attraction at a place can be detected by observing bearings from both ends of the line in the area. If fore bearing and back bearing of a line differ exactly by 180°, there is no local attraction at either station. But if this difference is not equal to 180°, then local attraction exists there either at one or both ends of the line.

Remedies

There are two common methods of correcting observed bearings of the lines taken in the area affected by Local Attraction. The first method involves ''correcting the bearing with the help of corrected included angles'' and the second method involves correcting the ''bearing of traverse from one correct bearing'' ( in which difference between fore bearing and back bearing is exactly equal to 180°) by the process of distribution of error to other bearings.Surveying by B.C. Punmia "Compass Surveying"

References

{{reflist Surveying Civil engineering