---
title: "Local attraction: errors and elimination"
subject: "Surveying I"
chapter: 4
key: "semester_3/surveying_i:local-attraction-sources-elimination"
source: https://www.bctengineer.com/q/semester_3/surveying_i/local-attraction-sources-elimination
site: BCT Engineering
---

# Local attraction: errors and elimination

Surveying I · Chapter 4: The Compass

- Asked in: 2062 Poush, 6 marks
- Video: [4.6 Compass Surveying: Local Attraction Correction by Bearings (Complete Numerical)](https://www.youtube.com/watch?v=JvOphyhwV44) — Civil Modules
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/local-attraction-sources-elimination
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4

## Question

What do you mean by local attraction in compass surveying? List the sources of error and also explain the method of elimination of local attraction.

## Short answer

Local attraction is deflection of the compass needle by nearby magnetic materials, giving constant error at a station. Sources include iron ore, steel structures, chains, keys. It is eliminated by starting from a line whose FB and BB differ by exactly 180° and applying corrections station by station, or by using included angles that are unaffected at the station.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Local attraction makes bearings at an affected station wrong by:
   A. Different random amounts
   B. Exactly 180°
   C. A constant amount
   D. Nothing, only distances change
   Answer: C. The needle is deflected equally for all lines from that station. (Key point: Definition)

2. Which is a source of local attraction carried by the surveyor?
   A. Wooden ranging rod
   B. Cotton thread
   C. Paper field book
   D. Steel tape, keys, mobile phone
   Answer: D. Small metal objects near the compass deflect the needle. (Key point: Sources)

3. Stations are free of attraction if:
   A. FB and BB differ by exactly 180°
   B. FB equals BB
   C. FB + BB = 360°
   D. FB is less than 180°
   Answer: A. A 180° difference means both ends are unaffected. (Key point: Detection)

4. Why do included angles help eliminate local attraction?
   A. Angles are measured by chain
   B. Both bearings at a station have the same error, which cancels in the angle
   C. Angles are always zero error
   D. Angles ignore the meridian entirely and use the sun
   Answer: B. Their difference is unaffected by a constant error at the station. (Key point: Included angle method)

## Related in Surveying I

- [Local attraction: AB 69°30′ compass traverse](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-ab-69-30-reordered) — Surveying I, chapter 4
- [Local attraction, bearings AB 305°00′ (EA 45°)](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-ea-305-00) — Surveying I, chapter 4, asked 2×
- [Local attraction, bearings AB 305°30′](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-ea-bb-44-00) — Surveying I, chapter 4, asked 2×
- [Local attraction, BC 79°30′ and DE 82°15′](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-ab-191-30-bc-79-30) — Surveying I, chapter 4
- [Bearings and local attraction explained](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Abearings-and-local-attraction-terms) — Surveying I, chapter 4, asked 2×
- [Local attraction in closed traverse PQRSTP](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-pqrstp-188-45) — Surveying I, chapter 4

## Related in other subjects

- [Types and sources of measurement errors](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Atypes-sources-of-errors) — Metrology, chapter 2
- [Phase and frequency error in local oscillator (DSB and SSB)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aphase-freq-error-lo-dsb-ssb) — Communication Systems, chapter 2, asked 2×
- [Thread errors and effect of pitch error](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ascrew-thread-errors-and-pitch-effect) — Metrology, chapter 12