--- title: "Obstacles in chain survey and remedies" subject: "Surveying I" chapter: 3 key: "semester_3/surveying_i:obstacles-chain-survey" source: https://www.bctengineer.com/q/semester_3/surveying_i/obstacles-chain-survey site: BCT Engineering --- # Obstacles in chain survey and remedies Surveying I · Chapter 3: Chain Survey - Asked in: 2066 Jestha, 7 marks; 2063 Baisakh, 6 marks; 2062 Baisakh, 6 marks; 2061 Baisakh, 8 marks (asked 4 times) - Video: [Problem 1 on Obstacles - Introduction to Surveying - Surveying 1](https://www.youtube.com/watch?v=FiV6DG2rDcI) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/surveying_i/obstacles-chain-survey - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-3 ## Question What are the obstacles in chain survey? Explain the methods of overcoming those obstacles. ## Short answer Obstacles are of three types. When vision is obstructed but chaining free, reciprocal ranging is used. When chaining is obstructed but vision free, perpendiculars and congruent triangles measure distances across ponds or rivers. When both are obstructed, equal perpendiculars bypass the obstacle to prolong the line. ## Answer The obstacles in chain surveying are of three types: 1. **Chaining free but vision obstructed** (e.g. a hill or rising ground). 2. **Vision free but chaining obstructed** (e.g. a pond or river). 3. **Both chaining and vision obstructed** (e.g. a building). #### 1. Chaining free, vision obstructed **Rising ground between two stations.** The end stations A and B are not intervisible, but the line can be chained over the hill. The line is ranged by **reciprocal ranging**: - Two persons stand at M1 and N1, points near the line such that M1 sees N1 and B, and N1 sees M1 and A. - The person at M1 directs N1 to a point N2 on the line M1B. Then N2 directs M1 to M2 on the line N2A. - The process is repeated until M, N, A and B are in one straight line. ```text M1 _______ N1 A ___/ hill \___ B ``` #### 2. Vision free, chaining obstructed **(a) Pond or lake on the line.** Erect two perpendiculars AC and BD of equal length at points A and B on the line, both on the **same side** of the line. Then CD is parallel and equal to AB, so measure CD. Alternatively, take a point O off the line and range A-O-C and B-O-D with OC = OA and OD = OB; the triangles OAB and OCD are congruent, so CD = AB. **(b) River.** To find the width AB: at A set out a perpendicular AC and mark D, its mid-point. At C set out CE perpendicular to AC (away from the river) and move along it until E, D and B are in a line. Then, by congruent triangles ABD and CED, $AB = CE$. ```text B (far bank) : ~~~ river ~~~ : A ---------- D ---------- C | | E (AD = DC; E, D, B in line; AB = CE) ``` #### 3. Both chaining and vision obstructed **Building on the line.** Take two points A and B on the line before the building. 1. At A and B erect perpendiculars AC and BD of equal length, long enough to clear the building. 2. Join C and D and extend CD past the building. Mark E and F on line CD beyond the building. 3. At E and F erect perpendiculars EG and FH equal to AC. G and H lie on the continuation of AB. 4. The hidden distance $BG = DE$. Measure DE and add it to the chainage of B. ```text C ------------ D ------------------ E ------------ F | | | | ---A--------------B----[ building ]----G--------------H---> ``` Alternatively, the line can be prolonged by trigonometry, using lines set out at known angles (for example $30^\circ$ and $40^\circ$) from the end of a perpendicular. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is the correct classification of obstacles in chain surveying? A. Chaining free vision obstructed; vision free chaining obstructed; both obstructed B. Natural, artificial and personal obstacles C. Obstacles to ranging only, to offsets only and to booking only D. Horizontal, vertical and inclined obstacles Answer: A. Obstacles are classed by whether chaining, vision or both are blocked. (Key point: Types of obstacles) 2. A hill lies between A and B so that they are not intervisible but the line can be chained over it. Which method is used? A. Erecting equal perpendiculars at A and B B. Reciprocal ranging C. Congruent triangles across the hill with CE = AB D. Prolonging the line with 30 and 40 degree angles Answer: B. This is chaining free, vision obstructed; reciprocal ranging fixes the line over the high ground. (Key point: Rising ground) 3. In the pond method, equal perpendiculars AC and BD are set at A and B. Why can CD be measured to get AB? A. CD is always longer than AB by a fixed amount B. CD is the hypotenuse of a right triangle with AB C. ACDB is a rectangle, so CD is parallel and equal to AB D. CD equals AC plus BD Answer: C. With AC = BD and both perpendicular to AB, ACDB is a rectangle and opposite sides are equal. (Key point: Pond method) 4. In the river method, D is the mid-point of AC, CE is perpendicular to AC and E, D, B are in line. What gives AB? A. AB = 2 x CE B. AB = AC C. AB = CE / 2 D. AB = CE by congruent triangles Answer: D. Triangles ABD and CED are congruent (AD = DC, right angles, vertical angles), so AB = CE. (Key point: River width) 5. For a building on the line, with equal perpendiculars at A, B and at E, F, what is the hidden length BG? A. BG = DE B. BG = AC C. BG = CD plus EF D. BG = EG Answer: A. CD is parallel to the line, so DEGB is a rectangle and BG = DE, which can be measured. (Key point: Building obstacle) ## Related in Surveying I - [Field book and obstacles in chain survey](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afield-book-and-obstacles) — Surveying I, chapter 3 - [Obstacles; field sketch for detail survey](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aobstacles-and-field-sketch) — Surveying I, chapter 3 - [Chain survey principle and operations](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-and-field-procedure) — Surveying I, chapter 3, asked 2× - [Principle of chain survey; types of offsets](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-and-offsets) — Surveying I, chapter 3