--- title: "When to use deflection angle traversing" subject: "Surveying II" chapter: 1 key: "semester_4/surveying_ii:deflection-angle-traverse-when-suggest" source: https://www.bctengineer.com/q/semester_4/surveying_ii/deflection-angle-traverse-when-suggest site: BCT Engineering --- # When to use deflection angle traversing Surveying II · Chapter 1: Traversing - Asked in: 2071 Magh, 6 marks - Video: [TRAVERSING BY METHOD OF DEFLECTION ANGLE // THEODOLITE TRAVERSING // THEODOLITE SURVEY](https://www.youtube.com/watch?v=z0qHSkzdvOQ) — BIG C - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/deflection-angle-traverse-when-suggest - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-1 ## Question When would you suggest a theodolite traversing by the method of deflection angles? Explain with neat sketches. ## Short answer Use deflection angles for open route surveys (roads, canals, pipelines) where the alignment changes gradually, deflections are small, and direct curve setting out data is needed. Also suitable for long narrow surveys and preliminary work, as the check ΣR – ΣL = 360° is simple. ## Answer A **deflection angle** is the angle between the prolongation of the previous line and the next line, measured to the right (R, clockwise) or to the left (L, anticlockwise). #### When it is suggested Traversing by deflection angles is preferred: - For **route and open traverses** such as roads, railways, canals, pipelines and transmission lines, where the lines run in one general direction and the deflections are small. - When the deflection angles are needed directly for setting out **curves** (the angle of deflection at the intersection point). - For **long, narrow route surveys** where the direction changes gradually, because small deflections are quick to measure, book and plot. - For **preliminary and reconnaissance work**, as the angle sum is easy to check for a closed route: $\Sigma R-\Sigma L=360^\circ$. It is not suggested for closed polygons with many short sides, where included angles are simpler and have a direct geometric check, $(2n-4)\times90^\circ$. #### Sketches and field procedure ```text Left deflection (L) C / / d (anticlockwise) A-------------B - - - - - - prolongation of AB Right deflection (R) A-------------B - - - - - - prolongation of AB \ d (clockwise) \ C ``` 1. Set up and level the theodolite at B. With the horizontal circle set to zero on face left, sight the back station A using the lower clamp. 2. **Plunge (transit)** the telescope so that it points along the prolongation of AB. 3. Loosen the upper clamp, sight C and read the angle. If C lies to the right of the prolongation, it is a right deflection; if to the left, a left deflection. 4. Repeat on the other face and take the mean. Each deflection is booked with R or L, and the bearing of a line is found from $$ \theta_{BC}=\theta_{AB}+d_R\quad\text{or}\quad\theta_{BC}=\theta_{AB}-d_L $$ Check for a closed traverse: $\Sigma d_R-\Sigma d_L=\pm360^\circ$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a deflection angle? A. The angle between the prolongation of the previous line and the next line B. The interior angle at a station C. The angle between north and a line D. The angle between two backsights Answer: A. It is measured from the prolongation of the preceding line to the next line, right or left. (Key point: Definition of deflection angle) 2. For which work is traversing by deflection angles suggested? A. Closed polygons with many small sides B. Route and open traverses such as roads, railways, canals and pipelines C. Small building plots D. Underground mines only Answer: B. Route surveys run in one general direction with small deflections, and the angles are needed directly for curve setting. (Key point: When deflection angles are preferred) 3. How is the prolongation of the back line established at station B? A. Backsight A, then plunge (transit) the telescope B. Sight C and turn 180° C. Read the compass needle D. Set the circle to 90° Answer: A. After sighting A, plunging the telescope points it along the extension of AB, from which C's deflection is read. (Key point: Field procedure) 4. What check applies to deflection angles of a closed traverse? A. ΣR - ΣL = 360° B. ΣR + ΣL = 180° C. ΣR = ΣL D. (2n-4) x 90° of the deflections Answer: A. The net deflection around a closed polygon is 360°, i.e. sum of right minus sum of left deflections = ±360°. (Key point: Closed traverse check) ## Related in Surveying II - [Rankine's deflection angle data, 145°, 5°](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Arankine-deflection-145-5deg-lc10) — Surveying II, chapter 6 - [Composite curve by deflection angle method](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Acomposite-curve-deflection-angle-method) — Surveying II, chapter 6, asked 5× - [Derive deflection angle of transition curve](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atransition-deflection-angle-derivation) — Surveying II, chapter 6, asked 3× - [Simple circular curve by deflection angle method](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Asimple-curve-deflection-angle-method-sketch) — Surveying II, chapter 6 - [Curve by tangential angle, deflection 48°30'](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atangential-angle-curve-radius-r-48) — Surveying II, chapter 6 - [Function of transition curve, deflection angle](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atransition-curve-function-deflection-angle) — Surveying II, chapter 6 ## Related in other subjects - [Open traverse ABCDEFG, deflection angles](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aopen-traverse-deflection-angles-40-45) — Surveying I, chapter 4, asked 2× - [Open traverse ABCDEFGH, deflection angles](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aopen-traverse-deflection-angles-81-45) — Surveying I, chapter 4, asked 2× - [Mean direction angles with VCR (125°25′20″)](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Amean-direction-traverse-abcda-vcr-125-25-20) — Surveying I, chapter 7 - [Measuring horizontal and vertical angles](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Ahorizontal-and-vertical-angle-methods) — Surveying I, chapter 7