---
title: "Local attraction: AB 69°30′ compass traverse"
subject: "Surveying I"
chapter: 4
key: "semester_3/surveying_i:local-attraction-ab-69-30-reordered"
source: https://www.bctengineer.com/q/semester_3/surveying_i/local-attraction-ab-69-30-reordered
site: BCT Engineering
---

# Local attraction: AB 69°30′ compass traverse

Surveying I · Chapter 4: The Compass

- Asked in: 2073 Shrawan, 8 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-ab-69-30-reordered
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4

## Question

The following bearing was observed in a compass traverse.

| Line | FB | BB |
|---|---|---|
| AB | 69°30′ | 246°30′ |
| BC | 191°30′ | 13°00′ |
| CD | 230°15′ | 53°00′ |
| DE | 262°45′ | 80°45′ |
| EA | 32°15′ | 210°30′ |

At which of these stations would local attraction be suspected? Find the corrected bearing of the lines.

## Short answer

No line has FB−BB=180°, so all stations are suspected. The sum of included angles is 537°30′; error –2°30′ is distributed equally, adding +30′ per angle. Taking BC line bearing (192°15′) as most reliable, the corrected FB bearings are AB 67°45′, BC 192°15′, CD 229°00′, DE 258°15′, EA 29°15′.

## Answer

The differences FB − BB are 183°00′, 178°30′, 177°15′, 182°00′ and 181°45′, so none of the lines is free from local attraction and **all stations A, B, C, D, E are suspected**. The included-angle method is used.

Local attraction at a station is detected by comparing the fore bearing (FB) and back bearing (BB) of each line: if there is no local attraction at either end, $\text{FB} - \text{BB} = 180^\circ$ exactly.

#### Step 1: Check each line

| Line | FB | BB | FB − BB | Remark |
|---|---|---|---|---|
| AB | 69°30′ | 246°30′ | 183°00′ | Differs by +3°00′ |
| BC | 191°30′ | 13°00′ | 178°30′ | Differs by −1°30′ |
| CD | 230°15′ | 53°00′ | 177°15′ | Differs by −2°45′ |
| DE | 262°45′ | 80°45′ | 182°00′ | Differs by +2°00′ |
| EA | 32°15′ | 210°30′ | 181°45′ | Differs by +1°45′ |

#### Step 2: Included (interior) angles

The included angle at a station uses the FB of the line leaving it and the BB of the line arriving at it, both observed at the same station, so local attraction cancels in the angle.

The sum of the interior angles should be $(2n-4)\times 90^\circ = 540^\circ$.

| Station | Angle between | Observed | Corrected |
|---|---|---|---|
| A | AE and AB | 141°00′ | 141°30′ |
| B | BA and BC | 55°00′ | 55°30′ |
| C | CB and CD | 142°45′ | 143°15′ |
| D | DC and DE | 150°15′ | 150°45′ |
| E | ED and EA | 48°30′ | 49°00′ |

Sum observed = 537°30′; error = −2°30′; correction per angle = +0°30′ (equal distribution, since each angle is measured with the same care).

No line has FB − BB = 180°. Line BC has the smallest difference (−1°30′), so it is the most reliable; its adopted bearing is the mean of the FB and (BB ± 180°) = 192°15′.

#### Step 3: Corrected bearings from the corrected angles

Rule used: FB of next line = BB of previous line − interior angle (traverse run clockwise). Starting from BC = 192°15′ (BB 12°15′): CD = 12°15′ − 143°15′ + 360° = 229°00′, and so on round the traverse.

| Line | Observed FB | Correction | Corrected FB | Observed BB | Corrected BB |
|---|---|---|---|---|---|
| AB | 69°30′ | −1°45′ | 67°45′ | 246°30′ | 247°45′ |
| BC | 191°30′ | +0°45′ | 192°15′ | 13°00′ | 12°15′ |
| CD | 230°15′ | −1°15′ | 229°00′ | 53°00′ | 49°00′ |
| DE | 262°45′ | −4°30′ | 258°15′ | 80°45′ | 78°15′ |
| EA | 32°15′ | −3°00′ | 29°15′ | 210°30′ | 209°15′ |

Check: corrected FB − corrected BB = 180° for every line.

#### Interior angles from the corrected bearings

| Station | Interior angle |
|---|---|
| A | 141°30′ |
| B | 55°30′ |
| C | 143°15′ |
| D | 150°45′ |
| E | 49°00′ |

Sum = 540°00′ (as required).

**Answer:** Corrected bearings (FB): AB = 67°45′, BC = 192°15′, CD = 229°00′, DE = 258°15′, EA = 29°15′.

## Check yourself

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

1. FB - BB values are 183°00′, 178°30′, 177°15′, 182°00′, 181°45′. Which stations are suspected?
   A. Only A and C
   B. Only the stations with positive differences
   C. None, since all are within 3°
   D. All of A, B, C, D and E
   Answer: D. No line has a difference of exactly 180°, so no station can be called free; all are suspected. (Key point: Suspected stations)

2. The observed angles sum to 537°30′ for a pentagon. What correction is applied to each angle?
   A. -0°30′
   B. +0°30′
   C. +2°30′
   D. +0°15′
   Answer: B. Required 540°, error -2°30′, so +150′/5 = +30′ per angle. (Key point: Angle error)

3. Line BC (FB 191°30′, BB 13°00′) is the most reliable. What is its adopted bearing?
   A. 191°30′
   B. 193°00′
   C. 192°15′
   D. 190°45′
   Answer: C. Mean of FB 191°30′ and BB + 180° = 193°00′ is 192°15′. (Key point: Adopted bearing)

## Related in Surveying I

- [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
- [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 ABCDEA by included angles](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-abcdea-included-angle) — 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 and true bearings, 1°30′E](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Acompass-closed-traverse-declination-1-30-east) — Surveying I, chapter 4
- [Local attraction, S 40°30′ W traverse](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-quadrantal-s40-30w) — Surveying I, chapter 4

## Related in other subjects

- [Traverse missing length CD, bearing EA](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-missing-cd-length-ea-bearing) — Surveying II, chapter 1
- [Closed traverse missing EA bearing, BC length](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aclosed-traverse-abcdea-missing-ea-bc) — Surveying II, chapter 1
- [Bearings of BC and CD in traverse ABCDE](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-abcde-bearings-bc-cd) — Surveying II, chapter 1, asked 2×
- [Missing bearings of BC and DE in traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-missing-bearings-bc-de) — Surveying II, chapter 1