What is local attraction How is it detected with the data of compass survey Dec2012 JuneJuly 2013

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What is local attraction How is it detected with the data of compass survey [Dec2012 JuneJuly-2013]

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By:Nikhil-bharadwaj

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LOCAL ATTRACTION
A magnetic meridian at a place is established by a magnetic needle which is uninfluenced by other attracting forces. However, sometimes, the magnetic needle may be attracted and prevented from indicating the true magnetic meridian when it is in proximity to certain magnetic substances. Local
attraction is a term used to denote any influence, such as the above, which prevents the needle from pointing to the magnetic north in a given locality. Some of the sources of local attraction are :
magnetite in the ground, wire carrying electric current, steel structures, railroad rails, underground iron pipes, keys, steel bowed spectacles, metal buttons, axes, chains, steel tapes etc., which may be lying on the ground nearby.
Detection of local attraction.
The local attraction at a particular place can be detected by observing the fore and back bearings of each line and finding its difference. If the difference between fore and back bearing is 1800 , it may be taken that both the stations are free from local attraction, provided there are no observational and instrumental errors. If the difference is other than 1800, the fore bearing should be measured again to find out whether the discrepancy is due to avoidable attraction from the articles on person, chains, tapes etc. it the difference still remains, the local attraction exists at one or both the stations.
Strictly speaking, the term local attraction does not include avoidable attraction due to things about the person or to other sources not connected with the place where the needle is read. Elimination of local attraction. If there is local attraction at a station. All the bearings measured at that place will be incorrect and the amount of error will be equal in all the bearings. There are two methods for eliminating the effects of local attraction.

First method: In this method, the bearings of the lines are calculated on the basis of the bearing of that line which has a difference of 1800
in its fore and back bearings. It is. However, assumed that
there are no observational and other instrumental errors. The amount and direction of error due to
local attraction at each of the affected station is found. If, however, there is no such line in which the
two bearings differ by 1800, the corrections should be made from the mean value of the bearing of
that line in which there is least discrepancy between the back sight and fore sight readings. If the bearings are expressed in quadrantal system, the corrections must be applied in proper direction. In 1st and 3rd quadrants, the numerical value of bearings increase in clockwise direction while they
increase in anti clockwise direction in 2nd and 4th quadrants. Positive corrections are applied clockwise and negative corrections counter clockwise.

Second method: This is more a general method and is based on the fact that though the bearings measured at a station may be incorrect due to local attraction, the included angel calculated from the bearings will be correct since the amount of error is the same for all the bearings measured at the
station. The included angles between the lines are calculated at all the stations. If the traverse is a close one, the sum of the internal included angles must be right angles. If there is any discrepancy in this, observational and instrumental errors also exist. Such error is distributed equally to all the
angles. Proceeding now with the line, the bearings of which differ by 1800
, the bearings of all other lines are calculated.

manjarimattur

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