--- title: "Local attraction, BC 79°30′ and DE 82°15′" subject: "Surveying I" chapter: 4 key: "semester_3/surveying_i:local-attraction-ab-191-30-bc-79-30" source: https://www.bctengineer.com/q/semester_3/surveying_i/local-attraction-ab-191-30-bc-79-30 site: BCT Engineering --- # Local attraction, BC 79°30′ and DE 82°15′ Surveying I · Chapter 4: The Compass - Asked in: 2068 Chaitra, 8 marks - 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-191-30-bc-79-30 - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4 ## Question Following are the bearings observed in a compass traverse survey. At what stations do you suspect local attraction? Correct them by applying a suitable correction method. | Line | FB | BB | |---|---|---| | AB | 191°30′ | 13°00′ | | BC | 79°30′ | 256°30′ | | CD | 32°15′ | 210°30′ | | DE | 262°45′ | 82°15′ | | EA | 230°15′ | 53°00′ | ## Short answer No line is free; all stations suspected. Included-angle method: interior angles sum 541°00′, error +1°00′, correction −0°12′ per angle. The most reliable line DE (smallest diff) is adopted as 262°30′. Corrected fore bearings: AB 188°36′, BC 74°54′, CD 30°27′, DE 262°30′, EA 230°18′. ## Answer No line is free (the differences are 178°30′, 183°00′, 181°45′, 180°30′ and 177°15′), so **all stations are suspected** of local attraction. Line DE has the smallest difference (0°30′), so D and E are the least affected; 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 | 191°30′ | 13°00′ | 178°30′ | Differs by −1°30′ | | BC | 79°30′ | 256°30′ | 183°00′ | Differs by +3°00′ | | CD | 32°15′ | 210°30′ | 181°45′ | Differs by +1°45′ | | DE | 262°45′ | 82°15′ | 180°30′ | Differs by +0°30′ | | EA | 230°15′ | 53°00′ | 177°15′ | Differs by −2°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 | 138°30′ | 138°18′ | | B | BA and BC | 66°30′ | 66°18′ | | C | CB and CD | 135°45′ | 135°33′ | | D | DC and DE | 52°15′ | 52°03′ | | E | ED and EA | 148°00′ | 147°48′ | Sum observed = 541°00′; error = +1°00′; correction per angle = −0°12′ (equal distribution, since each angle is measured with the same care). No line has FB − BB = 180°. Line DE has the smallest difference (+0°30′), so it is the most reliable; its adopted bearing is the mean of the FB and (BB ± 180°) = 262°30′. #### 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 = 262°30′ (BB 82°30′): EA = 82°30′ + 147°48′ = 230°18′, and so on round the traverse. | Line | Observed FB | Correction | Corrected FB | Observed BB | Corrected BB | |---|---|---|---|---|---| | AB | 191°30′ | −2°54′ | 188°36′ | 13°00′ | 8°36′ | | BC | 79°30′ | −4°36′ | 74°54′ | 256°30′ | 254°54′ | | CD | 32°15′ | −1°48′ | 30°27′ | 210°30′ | 210°27′ | | DE | 262°45′ | −0°15′ | 262°30′ | 82°15′ | 82°30′ | | EA | 230°15′ | +0°03′ | 230°18′ | 53°00′ | 50°18′ | Check: corrected FB − corrected BB = 180° for every line. #### Interior angles from the corrected bearings | Station | Interior angle | |---|---| | A | 138°18′ | | B | 66°18′ | | C | 135°33′ | | D | 52°03′ | | E | 147°48′ | Sum = 540°00′ (as required). **Answer:** Corrected bearings (FB): AB = 188°36′, BC = 74°54′, CD = 30°27′, DE = 262°30′, EA = 230°18′. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Differences from 180° are 1°30′, 3°00′, 1°45′, 0°30′ and 2°45′. Which stations are least affected? A. A and B B. B and C C. D and E (line DE) D. E and A Answer: C. DE has the smallest difference (0°30′), so D and E are least affected. (Key point: Least affected stations) 2. What is the observed interior angle at B (between BA and BC)? A. 66°30′ B. 56°30′ C. 76°30′ D. 243°30′ Answer: A. Angle = FB of BC - BB of AB = 79°30′ - 13°00′ = 66°30′. (Key point: Interior angle) 3. The observed angles sum to 541°00′. What correction is applied to each? A. +0°12′ B. -0°12′ C. -1°00′ D. -0°10′ Answer: B. Error +1°00′ = 60′ over 5 angles = 12′ each, subtracted. (Key point: Angle correction) ## 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°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, 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 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 ## Related in other subjects - [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× - [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×