Local Attraction | Methods for Correcting the bearings affected by Local Attraction
The deflection of a magnetic needle from its true position due to the presence of magnetic influencing material such as iron ore, magnetic rock, underground pipeline, electric cables, iron pipes, electric poles in its vicinity is called “Local Attraction”.
Methods of Correcting the bearings
There are two methods of correcting the bearings affected by local attraction:
- Included angle Method
- Error Computation
Included angle Method
In this method, the included angles of the traverse are calculated first, then starting from the line which is unaffected by local attraction and using the included angles, the corrected bearings of the traverse are computed.
Error Computation Method
In this method, the direction and the amount of local attraction at each survey station is determined.
Then starting from the line which is unaffected by local attraction, the corrected bearing of the traverse are computed.
This method is more accurate than the included angle method.
Hence it is adopted by most of the surveyors.
The substance of attraction with the surveyor such as buch of keys, iron buttons, spectacles etc.
It should be kept away from the compass.
The chain, arrows, clearing axe etc should be kept away from the compass.
If the entire area is magnetic, then compass surveying is not suitable in those places.
The correction to the survey lines are made starting from the line free from local attraction.