--- title: "Profile levelling; balancing of sight" subject: "Surveying I" chapter: 5 key: "semester_3/surveying_i:profile-leveling-balancing-of-sight" source: https://www.bctengineer.com/q/semester_3/surveying_i/profile-leveling-balancing-of-sight site: BCT Engineering --- # Profile levelling; balancing of sight Surveying I · Chapter 5: Leveling - Asked in: 2068 Baisakh, 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/profile-leveling-balancing-of-sight - Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-5 ## Question What is profile levelling? Explain the term "balancing of sight" in levelling operation. ## Short answer Profile levelling determines ground elevations along a centre line at regular intervals for longitudinal sections. Balancing of sight means keeping BS and FS distances equal; it eliminates errors due to collimation, curvature and refraction because these produce equal errors in both readings that cancel in the difference. ## Answer **Profile levelling (longitudinal sectioning)** is levelling along a given line, such as the centre line of a road, railway, canal or pipeline, to determine the elevations of the ground at regular intervals and at changes of slope, in order to draw the longitudinal section of the ground along the line. #### Procedure 1. Peg the centre line at equal intervals (10, 20 or 30 m) and note the chainage of each peg and of the breaks of slope. 2. Set up the level on firm ground within about 100 m of a BM and take a BS on the BM. 3. Hold the staff on each peg within reach along the line and take an intermediate sight (IS); take an FS on a change point when the level has to be shifted, and a BS on that point from the new position. 4. Close the levelling on a BM (or the starting BM) to check the work. 5. Reduce the levels (HI or rise and fall method), apply the arithmetic check, and distribute any closing error. 6. Plot the RLs against the chainage (with the vertical scale larger than the horizontal) to get the profile. ```text RL | ____ |__/ \__ ___ <- ground profile | \___/ +------------------- chainage ``` #### Balancing of sight Balancing of sights means keeping the **backsight distance equal to the foresight distance** from each instrument position. - The instrument is placed midway between the BS and FS points as far as possible. - The error due to the **line of collimation not being horizontal** is proportional to the sight distance, and the errors due to **curvature of the earth** and **atmospheric refraction** are proportional to its square. For equal sights these errors are equal in the BS and FS readings and cancel when the difference is taken. - When equal distances cannot be kept at each setup (for example on steep ground), the sum of the BS distances is kept equal to the sum of the FS distances. This still cancels the collimation error; the curvature and refraction errors are negligible for ordinary sight lengths. It is also followed in fly levelling and in differential levelling generally. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the purpose of profile levelling? A. To find ground elevations along a line at regular intervals and slope changes to draw a longitudinal section B. To find the area of a plot C. To find the horizontal angle between two lines D. To fix the boundary of a field Answer: A. Profile levelling gives ground RLs along a centre line such as a road or canal to plot the longitudinal section. (Key point: Profile levelling purpose) 2. How is the profile plotted? A. RL against time B. RL against chainage, with a larger vertical scale than horizontal C. RL against chainage, with equal scales D. Chainage against bearing Answer: B. A larger vertical scale exaggerates the small height differences so the relief is visible. (Key point: Plotting scales) 3. What does balancing of sights mean? A. Keeping the staff the same height at BS and FS B. Taking the same readings from both ends C. Keeping the BS distance equal to the FS distance from each instrument position D. Taking equal numbers of BS and IS Answer: C. The instrument is placed midway between BS and FS points so the sight lengths are equal. (Key point: Balancing of sight meaning) 4. Why does balancing of sights improve accuracy? A. It removes personal reading mistakes B. It removes the need for change points C. It makes the staff vertical D. Collimation, curvature and refraction errors are proportional to distance, so equal sights give equal errors that cancel in the difference Answer: D. Equal errors in BS and FS cancel when the difference of readings is taken. (Key point: Errors cancelled) ## Related in Surveying I - [Levelling methods; errors eliminated by balancing](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprinciple-methods-leveling-balancing-sight) — Surveying I, chapter 5, asked 2× - [Profile and cross-section levelling](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprofile-and-cross-section-leveling) — Surveying I, chapter 5, asked 10× - [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 - [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 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 ## Related in other subjects - [Balancing traverse, methods, independent coordinates](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Abalancing-traverse-methods-independent-coordinates) — Surveying II, chapter 1 - [TTL voltage profile; TTL-to-CMOS interface](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-voltage-profile-ttl-cmos-interface) — Digital Logic, chapter 10 - [Equivalent inertia, speed profile and quadrants (Fig 2b)](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aequivalent-inertia-speed-profile-quadrants) — Utilization of Electrical Energy, chapter 2