LOCAL ATTRACTION OF COMPASS SURVEYING

The magnetic needle does not indicates the north direction, when it is under the influence of attractive bodies in its surroundings, such a disturbing influence is termed as “Local attraction”. The disturbing magnetic substances may be iron ore, steel structures, rails, electric cables carrying current, lamp posts, fences and materials such as bunch of keys, knife, spectacles etc. with the surveyor.
Detection of Local Attraction:
Unless the local attraction at a place is found-out and corrected, the bearings taken with a compass are not correct. If the difference of the fore and back bearings of the line is exactly 180°, then there is no local attraction.
If the difference in FB and B.B of a line is not equal to 180°, then needle is said to be offered by local attraction, provided there is no instrumental error

APPLICATION OF CORRECTION FOR LOCAL ATTRACTION:

There are two methods for the application of correction, they are discussed below.
1. Direct Method: From the given table of observations, the unaffected line is first founded. This method is simple and generally employed.
2. Interior Angle Method: In this method first, all the interior angles of a traverse are calculated from the observed bearings. Then an angular check is applied. The sum of the interior angles should be equal to (2n-4) x 90°, where n is the number of sides of the traverse
Then starting from the unaffected line, the bearings of all the lines are corrected by using the corrected interior angles. This method is very laborious and hence generally not employed.

Note: In case, all the lines of the traverse are found to be affected by local attraction, the line with minimum error is identified. The FB and B.B of this line are adjusted by distributing the error equally. Then starting from this adjusted line, the fore and back bearings of the other lines are corrected.