--- title: "Local attraction in closed traverse ABCDEFA" subject: "Surveying I" chapter: 4 key: "semester_3/surveying_i:local-attraction-abcdefa-216-30" source: https://www.bctengineer.com/q/semester_3/surveying_i/local-attraction-abcdefa-216-30 site: BCT Engineering --- # Local attraction in closed traverse ABCDEFA Surveying I · Chapter 4: The Compass - Asked in: 2074 Asoj, 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-abcdefa-216-30 - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4 ## Question The bearings of a closed traverse ABCDEFA are given as follows. Find the stations affected by local attraction and correct them if necessary. | Line | Fore Bearing | Back Bearing | |---|---|---| | AB | 216°30′ | 36°10′ | | BC | 135°55′ | 316°25′ | | CD | 81°30′ | 260°30′ | | DE | 321°10′ | 141°20′ | | EF | 246°20′ | 66°50′ | | FA | 299°20′ | 119°00′ | ## Short answer All lines have FB−BB differing from 180°, indicating local attraction at every station. Included angle sum is 720°30′, so +0°30′ error is corrected by –5′ per angle; adopting DE as most reliable (321°15′), corrected FB bearings: AB 216°00′, BC 135°40′, CD 80°40′, DE 321°15′, EF 246°10′, FA 298°35′. ## Answer Every line differs from 180° (by 0°20′, 0°30′, 1°00′, 0°10′, 0°30′ and 0°20′), so no station can be called free with certainty, and all stations are suspected of local attraction. Line DE has the smallest difference, so D and E are the least affected stations; the bearings are corrected using the included angles. 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 | 216°30′ | 36°10′ | 180°20′ | Differs by +0°20′ | | BC | 135°55′ | 316°25′ | 179°30′ | Differs by −0°30′ | | CD | 81°30′ | 260°30′ | 181°00′ | Differs by +1°00′ | | DE | 321°10′ | 141°20′ | 179°50′ | Differs by −0°10′ | | EF | 246°20′ | 66°50′ | 179°30′ | Differs by −0°30′ | | FA | 299°20′ | 119°00′ | 180°20′ | Differs by +0°20′ | #### 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 $(2n-4)\times 90^\circ = 720^\circ$. | Station | Angle between | Observed | Corrected | |---|---|---|---| | A | AF and AB | 97°30′ | 97°25′ | | B | BA and BC | 99°45′ | 99°40′ | | C | CB and CD | 125°05′ | 125°00′ | | D | DC and DE | 60°40′ | 60°35′ | | E | ED and EF | 105°00′ | 104°55′ | | F | FE and FA | 232°30′ | 232°25′ | Sum observed = 720°30′; error = +0°30′; correction per angle = −0°05′ (equal distribution, since each angle is measured with the same care). No line has FB − BB = 180°. Line DE has the smallest difference (−0°10′), so it is the most reliable; its adopted bearing is the mean of the FB and (BB ± 180°) = 321°15′. #### Step 3: Corrected bearings from the corrected angles Rule used: FB of next line = BB of previous line + interior angle (traverse run anticlockwise). Starting from DE = 321°15′ (BB 141°15′): EF = 141°15′ + 104°55′ = 246°10′, and so on round the traverse. | Line | Observed FB | Correction | Corrected FB | Observed BB | Corrected BB | |---|---|---|---|---|---| | AB | 216°30′ | −0°30′ | 216°00′ | 36°10′ | 36°00′ | | BC | 135°55′ | −0°15′ | 135°40′ | 316°25′ | 315°40′ | | CD | 81°30′ | −0°50′ | 80°40′ | 260°30′ | 260°40′ | | DE | 321°10′ | +0°05′ | 321°15′ | 141°20′ | 141°15′ | | EF | 246°20′ | −0°10′ | 246°10′ | 66°50′ | 66°10′ | | FA | 299°20′ | −0°45′ | 298°35′ | 119°00′ | 118°35′ | Check: corrected FB − corrected BB = 180° for every line, and working round the traverse returns to DE = 321°15′. **Answer:** Corrected bearings (FB): AB = 216°00′, BC = 135°40′, CD = 80°40′, DE = 321°15′, EF = 246°10′, FA = 298°35′. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Differences from 180° are 0°20′, 0°30′, 1°00′, 0°10′, 0°30′ and 0°20′. Which line is least affected? A. AB B. DE C. CD D. FA Answer: B. DE has the smallest difference (0°10′), so D and E are least affected. (Key point: Most reliable line) 2. For the six-sided traverse ABCDEF, what should the interior angles total? A. 720° B. 540° C. 900° D. 360° Answer: A. (2n-4) x 90° = (12-4) x 90° = 720°. (Key point: Angle sum) 3. The observed angles sum to 720°30′. What correction is applied to each angle? A. +0°05′ B. -0°30′ C. -0°05′ D. -0°10′ Answer: C. Error +0°30′ = 30′, divided over 6 angles is 5′ each, subtracted. (Key point: Equal distribution) ## Related in Surveying I - [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: 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 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, 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, 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 - [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 - [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×