---
title: "Points to consider in fly levelling"
subject: "Surveying I"
chapter: 5
key: "semester_3/surveying_i:fly-leveling-points-to-consider"
source: https://www.bctengineer.com/q/semester_3/surveying_i/fly-leveling-points-to-consider
site: BCT Engineering
---

# Points to consider in fly levelling

Surveying I · Chapter 5: Leveling

- Asked in: 2074 Asoj, 4 marks
- Video: [Fly levelling!Levelling! type of levelling ! quickest method leveling ! upsssc je ! ssc je ! AE!JE](https://www.youtube.com/watch?v=qhCQ8t8WF7w) — NARAYANA COACHING CLASSES
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/fly-leveling-points-to-consider
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-5

## Question

State the points to be considered in fly levelling.

## Short answer

Fly levelling uses only BS and FS. Points: keep BS and FS distances equal; limit sights to 60–100 m; use firm change points and vertical staff; start and close on benchmarks; check ΣBS−ΣFS = last RL−first RL; ensure instrument adjustment; closing error must fall within ±24√K mm (ordinary) or ±12√K mm (accurate).

## Answer

Fly levelling is levelling in which only BS and FS readings are taken, to find the elevation of a distant BM or to check levels previously established. The points to be considered are:

1. **Balance the sights:** keep the BS and FS distances approximately equal to remove the errors of collimation, curvature and refraction.
2. **Sight length:** limit the length of sight to 60–100 m (usually not more than 100 m) and avoid very short sights; it should be such that the staff can be read with confidence.
3. **Change points:** choose firm, well-defined points (such as pegs with a nail or a stone) and keep the staff on the same spot between the FS and the BS; the staff should be held vertical (use a staff bubble).
4. **Starting and closing:** start from a BM of known RL and close on another BM, or return to the starting BM (a loop), to check the work.
5. **Readings** should be taken to the nearest millimetre, entered at once in the field book and checked: $\Sigma BS - \Sigma FS = $ last RL $-$ first RL.
6. **Instrument:** it must be well adjusted (two-peg test); the bubble should be centred before each reading; avoid sighting at the bottom (< 0.5 m) of the staff because of refraction, and avoid reading in strong wind or heat shimmer.
7. **Closing error** must be within the permissible limit, $\pm C\sqrt{K}$ mm (K in km), for example $\pm 24\sqrt{K}$ for ordinary work and $\pm 12\sqrt{K}$ for more accurate work; if exceeded, repeat the levelling.
8. The work should be done **quickly**, in one direction, with the same instrument and observer, and in the **same weather conditions**.

## Check yourself

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

1. In fly levelling, which readings are taken?
   A. Only BS and FS
   B. BS, IS and FS at every peg
   C. Only IS
   D. Only FS
   Answer: A. Fly levelling is a quick check or transfer of a BM using only BS and FS at change points. (Key point: Fly levelling definition)

2. Why are BS and FS distances kept approximately equal in fly levelling?
   A. To keep the staff vertical
   B. To remove collimation, curvature and refraction errors
   C. To save time in booking
   D. To avoid change points
   Answer: B. Equal sights make these distance-proportional errors cancel. (Key point: Balance the sights)

3. How is the work checked in fly levelling?
   A. By comparing with a map
   B. No check is possible
   C. By closing on another BM or returning to the starting BM
   D. By repeating only the last reading
   Answer: C. The closing error against a known RL tells whether the work is within the limit. (Key point: Checking closure)

4. If the closing error exceeds the permissible limit of +-C sqrt(K) mm, what should be done?
   A. Distribute it equally and continue
   B. Ignore it
   C. Discard the last BM
   D. Repeat the levelling
   Answer: D. An error beyond the limit means a mistake or poor work; the levelling must be repeated. (Key point: Closing error limit)

## Related in Surveying I

- [Fly levelling, BM 1470 m; set 4 pegs](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afly-level-set-4-pegs-1470-rising-1-in-200) — Surveying I, chapter 5
- [Fly levelling, BM 1280 m; falling gradient](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afly-level-set-4-pegs-1280-falling-1-in-200) — Surveying I, chapter 5
- [Fly levelling with inverted staff, BM 1290.500](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afly-level-inverted-staff-1290-500) — Surveying I, chapter 5
- [Fly levelling for road pegs, BM 3010 m](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afly-level-road-pegs-3010m) — Surveying I, chapter 5
- [Fly levelling from BM RL 1291.610](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afly-level-bm-1291-610) — Surveying I, chapter 5
- [Reciprocal levelling: RL of A, B at 1160.18 m](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Areciprocal-leveling-rl-b-1160-18) — Surveying I, chapter 5, asked 2×

## Related in other subjects

- [Points to consider when establishing traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-establishing-points-considered) — Surveying II, chapter 1
- [Points to consider when measuring earthwork](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Apoints-measuring-earthwork) — Estimating and Costing, chapter 2