Which is the efficient levelling method when the distance between two points is NOT within the visible range of the level instrument?
Correct Answer: D. Differential levelling
📚 Detailed Explanation: Differential Levelling for Points Beyond Visible Range
Why D (Differential levelling) is correct: When the distance between two points exceeds the visible range of a level, or when obstacles block a direct line of sight, differential levelling is used. The instrument is set up at multiple sequential positions, moving through intermediate change points, until the distant target is reached. Each setup extends the reach by one instrument sight distance.
Comparison of Levelling Methods
| Method | When Used | Key Characteristic |
|---|---|---|
| Simple Levelling | Two points visible from a single setup | One BS and one FS; no change points |
| Differential Levelling | Points too far apart for single setup; beyond visible range | Multiple setups with change points; BS and FS at each CP |
| Fly Levelling | Rapid reconnaissance; BM verification | Only BS and FS logged; intermediate sights omitted for speed |
| Profile Levelling | Longitudinal section along a route | IS at regular chainages; both distance and elevation recorded |
| Reciprocal Levelling | Wide obstacle (river, valley); cannot set instrument at midpoint | Readings from both sides; averages cancel errors |
| Block Levelling | Contouring of an area (grid levelling) | IS at grid intersections; produces spot heights over area |
- Differential levelling is used when the two points are beyond the visible range of a single instrument setup.
- Multiple change points allow the survey to “step” over any distance, regardless of terrain.
- Arithmetic check: ΣBS − ΣFS = Last RL − First RL (must be satisfied for accuracy verification).