Local attraction: errors and elimination

Local attraction is the deflection of the magnetic needle from the magnetic meridian caused by magnetic material near the compass. It makes the observed bearings at that station wrong by a constant amount.

Sources

• Natural: iron-ore deposits, magnetite, rocks and soil rich in iron.
• Artificial: steel rails, iron pipes, steel girders and bridges, reinforcing bars, electric poles and cables, iron fences, vehicles.
• Small objects carried by the surveyor: steel chain or tape, keys, knife, iron-shod tools, mobile phone, spectacles with metal frame.

Detection

The FB and BB of each line are compared. If they differ by exactly $180^\circ$, both stations are free. If not, one or both stations are affected.

Elimination

1. Before the survey: keep metal objects away; select stations away from known sources of attraction; take bearings of the line from both ends.
2. First method (correction of stations): find a line with FB − BB = $180^\circ$; its stations are free. Starting from it, compare the BB of the next line with the correct value; the difference at the next station is the correction applied to all bearings observed at that station. Continue to the other stations.
3. Second method (included angles): the angle at a station is not affected by local attraction there, because both bearings are in error by the same amount. Compute all included angles, correct their sum to $(2n - 4) \times 90^\circ$, fix the bearing of one reliable line, and calculate the others from the corrected angles.
4. In an open traverse, use the angle method or take back-sights and resights to check.