This article provides a note on local attraction along with its remedies and examples.
The magnetic needle is disturbed from its normal position if it is under the influence of external attractive forces called the sources of local attraction. Such a disturbing influence is known as local attraction. The term is also used to denote the amount of deviation of the needle from its normal position.
The local attractive sources may be neutral such as the masses of magnetic rock or iron ore etc. and artificial such as proximity of steel structures, rails, iron-pipes, current carrying wires etc. The iron made instruments such as chains, arrows, ranging rods and other things such as bunch of keys, knife, iron-buttons etc. should also be kept away from the compass at a safe distance apart.
Local attraction at a place can be detected by observing bearings, from both ends, of a line in the area. If the fore and back bearings of the line differ exactly by 180°, there is no local attraction at either station provided instrumental and observational errors are eliminated. But if this difference is not equal to 180°, then local attraction exists there either at one or at both ends of the line.
ADVERTISEMENTS:
Remedies for Local Attraction:
There are the following two methods of correcting the observed bearings of the lines taken in the area affected by the local attraction:
(i) The error due to local attraction being same for each of the bearings observed at an affected station, the included angle calculated from the bearings taken at the same station will be correct, even though, the station is affected by the local attraction. Starting from the unaffected line and using these included angles, the correct bearings of the successive lines are calculated as already explained.
(ii) The observed bearings are corrected by applying correction to the stations affected by the local attraction. For this, error at each of the station is found and then starting from an unaffected bearing, the bearings of the successive lines are adjusted by applying corrections to them.
ADVERTISEMENTS:
Note:
The magnitude and nature of error at a station due to local attraction can be determined by drawing a sketch of the observed and correct bearings of a line at that station. If the error is negative then the correction is positive, and vice versa, the magnitude of the two remaining equal to each other.
If the bearings are given in the whole circle system, then remember the following rule for finding the nature of error:
Rule:
ADVERTISEMENTS:
If at a station, observed bearing of a line is more than that of its correct one, then the error at that station is +ve and the correction is -ve and vice versa.
Examples on Local Attraction:
Example 1:
The following are the observed bearings of the lines of a traverse ABCD taken with a compass in a place where local attraction was suspected:
Find the correct bearing of the lines.
Solution:
On examining, it is found that fore and back bearings of line AB differ exactly by 180°. Stations A and B are, therefore free from local attraction. Consequently, the bearing observed at A and B are correct.
As the observed back bearing of BC is less than the corrected one, therefore, the error at C is -ve and the correction is +ve.
As the observed back bearing of CD is more than the corrected one, therefore, the error at D is +ve and the correction is -ve.
The result may be tabulated as shown below:
Example 2:
The following are the observed fore and back bearings of lines of a closed traverse. Correct them where necessary for local attraction:
Solution:
On examining, it is found that fore and back bearings of line BC differ exactly by 180°. Stations B and C are therefore, free from local attraction. Consequently, bearings taken at B and C are correct.’
As the observed back bearing of CD is less than the corrected one, therefore, the error at D is -ve and the correction is +ve.
The result may be tabulated as shown below:
