---
title: "Fly levelling from BM RL 1291.610"
subject: "Surveying I"
chapter: 5
key: "semester_3/surveying_i:fly-level-bm-1291-610"
source: https://www.bctengineer.com/q/semester_3/surveying_i/fly-level-bm-1291-610
site: BCT Engineering
---

# Fly levelling from BM RL 1291.610

Surveying I · Chapter 5: Leveling

- Asked in: 2064 Jestha, 10 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/fly-level-bm-1291-610
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-5

## Question

In running a fly level from a bench mark of RL 1291.610 m, the following readings were obtained. Back Sight: 2.543 (B.M.), 2.094, 1.916, 2.725. Fore Sight: 1.645, 1.436, 0.956, 1.855. Prepare a level book and compute reduced levels with necessary checks.

## Short answer

Using the height of instrument method with rise and fall check, the reduced levels are: TP1 = 1292.508 m, TP2 = 1293.166 m, TP3 = 1294.126 m, and the last point = 1294.996 m. The arithmetic check gives ΣBS − ΣFS = 3.386 m, matching the difference between last and first RL.

## Answer

**Method:** a fly level uses only back sights and fore sights (no intermediate sights), each fore sight and the next back sight being on the same turning point. Height of instrument (HI) method is used with a check by rise and fall.

Taking BM = 1291.610 m, the readings are booked in four set-ups as below.

| Point | BS | IS | FS | HI | Rise | Fall | RL (m) | RL by HI (m) |
|---|---|---|---|---|---|---|---|---|
| BM | 2.543 |  |  | 1294.153 |  |  | 1291.610 | 1291.610 |
| TP1 | 2.094 |  | 1.645 | 1294.602 | 0.898 |  | 1292.508 | 1292.508 |
| TP2 | 1.916 |  | 1.436 | 1295.082 | 0.658 |  | 1293.166 | 1293.166 |
| TP3 | 2.725 |  | 0.956 | 1296.851 | 0.960 |  | 1294.126 | 1294.126 |
| Last |  |  | 1.855 |  | 0.870 |  | 1294.996 | 1294.996 |

HI at each row with BS = RL of that point + BS; RL of the next point = HI - FS.

#### Arithmetic checks
$$
\begin{aligned}
\Sigma BS - \Sigma FS &= 9.278 - 5.892 = 3.386\\
\Sigma \text{Rise} - \Sigma \text{Fall} &= 3.386 - 0.000 = 3.386\\
\text{Last RL} - \text{first RL} &= 1294.996 - 1291.610 = 3.386
\end{aligned}
$$

All checks agree.

**Answer:** RLs of the turning points = 1292.508, 1293.166, 1294.126 m, and the last point = **1294.996 m**.

## Check yourself

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

1. What readings are used in fly levelling?
   A. BS and FS only, no intermediate sights
   B. BS, IS and FS
   C. Only FS
   D. Only IS
   Answer: A. Fly levelling transfers levels using only back sights and fore sights on turning points. (Key point: Fly levelling)

2. What is HI at BM?
   A. 1291.610 - 2.543 = 1289.067 m
   B. 1291.610 + 1.645 = 1293.255 m
   C. 2.543 m
   D. 1291.610 + 2.543 = 1294.153 m
   Answer: D. HI = RL + BS = 1294.153 m. (Key point: HI at BM)

3. FS at TP1 is 1.645. What is its RL?
   A. 1291.610 m
   B. 1290.508 m
   C. 1295.798 m
   D. 1292.508 m
   Answer: D. RL = HI - FS = 1294.153 - 1.645 = 1292.508 m, a rise of 0.898 m. (Key point: RL of TP1)

4. What equal value comes out of the checks?
   A. -3.386 m
   B. +9.278 m
   C. +3.386 m
   D. 0.000 m
   Answer: C. 9.278 - 5.892 = 3.386; the rises total 3.386 with no falls; 1294.996 - 1291.610 = 3.386. (Key point: Arithmetic check)

## Related in Surveying I

- [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, 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 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 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
- [Level book: profile levelling readings](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprofile-leveling-readings-3rd-7th-9th) — Surveying I, chapter 5
- [Level book, staff readings at B and C (1251.250)](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alevel-book-15m-b-c-rl-1250-800-1251-250) — Surveying I, chapter 5, asked 3×

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