Let us go through some of the FAQs related to local attractions in compass surveying.
1. What is the local attraction in compass surveying?
While surveying, the magnetic needle of the compass gets disturbed due to the external influence of forces or objects. This prevents the needle from pointing correctly toward the magnetic north & creates an error.
Such a disturbing influence of external forces or objects over the compass needle is said to be a local attraction.
2. What are the different sources of local attraction?
Following are some of the sources of local attractions.
1. Cables or wires carrying electric currents.
2. Underground metal objects like pipelines for water & conduits.
3. Nearby railway tracks.
4. The earth layers that contain natural iron ores like magnetite, pyrite, hematite, etc.
5. Steel structures & objects.
6. Survey equipment like chains, rods, steel tapes, etc.
7. Steel materials present in our pockets such as key chains, coins, or metal buttons.
3. How we can detect local attractions?
After observing the fore bearing & back bearing of the survey line, we have to calculate the difference between the FB & BB observed from a particular station.
If the difference is 180° then we can conclude that there is no local attraction for that station & hence no correction is needed.
If the difference is ±180° then we can conclude that there is a local attraction for that station & hence correction is needed.
In the above-given drawing,
If ∠𝜶 ∓ ∠β = 180° then there is no local attraction in station A
If ∠𝜶 ∓ ∠β ≠ 180° then there is a local attraction in station A
4. What are the different methods to correct or eliminate the local attraction?
Two methods are used to correct the errors formed by local attractions.
1. Correction in the bearings affected by local attraction.
2. Correction in the interior angles formed by the closed traverse.
To know, how to correct the bearings affected by local attraction, Go through the article 👇
👀. Correction of bearings affected by local attraction.
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