---
title: "Local attraction, bearings AB 305°30′"
subject: "Surveying I"
chapter: 4
key: "semester_3/surveying_i:local-attraction-ea-bb-44-00"
source: https://www.bctengineer.com/q/semester_3/surveying_i/local-attraction-ea-bb-44-00
site: BCT Engineering
---

# Local attraction, bearings AB 305°30′

Surveying I · Chapter 4: The Compass

- Asked in: 2069 Chaitra, 8 marks
- Video: [local attraction problem in surveying || compass survey|| survey engineering|| #local_attraction](https://www.youtube.com/watch?v=i_3QTIOjHek) — Aim 2 Civil Class
- 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-ea-bb-44-00
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4

## Question

The following bearings were observed in a compass traverse.

| Line | AB | BC | CD | DE | EA |
|---|---|---|---|---|---|
| FB | 305°30′ | 75°30′ | 115°30′ | 166°30′ | 225°00′ |
| BB | 125°30′ | 254°30′ | 297°30′ | 345°00′ | 44°00′ |

At which stations do you suspect local attraction? Find the correct bearings of all the lines.

## Short answer

Line AB is free but direct correction fails to close, so the included-angle method is used. Observed interior angles sum 538°30′, error −1°30′, corrected by +0°18′ each. Starting from AB 305°30′, corrected fore bearings are AB 305°30′, BC 75°12′, CD 115°54′, DE 164°36′, EA 224°18′.

## Answer

Line AB has FB − BB = 180°00′, so stations A and B are free. The differences for the other lines are BC +1°00′, CD −2°00′, DE +1°30′ and EA +1°00′, so **C, D and E are suspected** (A and B are not). Correcting the stations one by one starting from AB does not close: going round the traverse, the correction found for A comes out as +1°30′ although A is free. So 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 | 305°30′ | 125°30′ | 180°00′ | Free, difference = 180° |
| BC | 75°30′ | 254°30′ | 181°00′ | Differs by +1°00′ |
| CD | 115°30′ | 297°30′ | 178°00′ | Differs by −2°00′ |
| DE | 166°30′ | 345°00′ | 181°30′ | Differs by +1°30′ |
| EA | 225°00′ | 44°00′ | 181°00′ | Differs by +1°00′ |

#### 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 | 98°30′ | 98°48′ |
| B | BA and BC | 50°00′ | 50°18′ |
| C | CB and CD | 139°00′ | 139°18′ |
| D | DC and DE | 131°00′ | 131°18′ |
| E | ED and EA | 120°00′ | 120°18′ |

Sum observed = 538°30′; error = −1°30′; correction per angle = +0°18′ (equal distribution, since each angle is measured with the same care).

Line AB has FB − BB = 180°, so its bearing is free from local attraction and is taken as the starting line.

#### 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 AB = 305°30′ (BB 125°30′): BC = 125°30′ − 50°18′ = 75°12′, and so on round the traverse.

| Line | Observed FB | Correction | Corrected FB | Observed BB | Corrected BB |
|---|---|---|---|---|---|
| AB | 305°30′ | 0 | 305°30′ | 125°30′ | 125°30′ |
| BC | 75°30′ | −0°18′ | 75°12′ | 254°30′ | 255°12′ |
| CD | 115°30′ | +0°24′ | 115°54′ | 297°30′ | 295°54′ |
| DE | 166°30′ | −1°54′ | 164°36′ | 345°00′ | 344°36′ |
| EA | 225°00′ | −0°42′ | 224°18′ | 44°00′ | 44°18′ |

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

#### Interior angles from the corrected bearings

| Station | Interior angle |
|---|---|
| A | 98°48′ |
| B | 50°18′ |
| C | 139°18′ |
| D | 131°18′ |
| E | 120°18′ |

Sum = 540°00′ (as required).

**Answer:** Corrected bearings (FB): AB = 305°30′, BC = 75°12′, CD = 115°54′, DE = 164°36′, EA = 224°18′.

## Check yourself

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

1. Line AB has FB - BB = 180°. Which stations are suspected?
   A. C, D and E
   B. A, B and C
   C. B, C and D
   D. All five stations
   Answer: A. A and B are free; the other lines differ from 180°, so C, D and E are suspected. (Key point: Suspected stations)

2. Why is the included-angle method used here?
   A. The compass has no least count
   B. Station corrections always fail for pentagons
   C. AB is the only line with a BB
   D. Correcting stations forward and backward from AB gives different values (closing difference 1°30′)
   Answer: D. The two directions of station-by-station correction do not agree, so the angle method with equal distribution is used. (Key point: Why included angles)

3. The observed angles total 538°30′. What is added to each angle?
   A. +0°30′
   B. +0°18′
   C. -0°18′
   D. +1°30′
   Answer: B. Error is -1°30′ = 90′; 90′/5 = 18′ added to each. (Key point: Angle correction)

## Close variants set in other exams

- [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×

## 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, 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 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 and true bearings, 7°W](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-true-bearings-7w) — 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
- [Local attraction with quadrantal bearings](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-quadrantal-bearings-s11-30w) — Surveying I, chapter 4

## Related in other subjects

- [Bearings of DE and EA in traverse ABCDE](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-abcde-bearings-de-ea) — Surveying II, chapter 1, asked 2×
- [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×