Fly levelling is primarily used for:
Correct Answer: B. Reconnaissance
📚 Detailed Explanation: Fly Levelling Is Used for Reconnaissance
Why B (Reconnaissance) is correct: Fly levelling is a quick, rough survey of a route to check the general feasibility of the terrain before detailed levelling begins. It is not a precise survey — it is used for preliminary investigation (reconnaissance).
Types of Levelling and Their Purposes
| Type of Levelling | Purpose | Accuracy Level |
|---|---|---|
| Fly levelling | Reconnaissance — quickly carry levels from one BM to a distant BM; check feasibility of a route before precise work begins | Low (rough) |
| Check levelling | To verify results of a previously completed levelling survey (independent repeat) | Medium |
| Profile (longitudinal section) levelling | To determine the elevations of points along a line (centre line of road, canal, etc.) | Medium to high |
| Precise levelling | For establishing primary benchmarks and geodetic control; uses special instruments, procedures, and rigid error limits | Very high |
| Cross-section levelling | To determine ground elevations transverse to a centre line | Medium |
Key Distinction: Fly levelling is done before the main survey. Its purpose is reconnaissance — to understand the lie of the land and identify significant obstacles. Check levelling is done after the main survey to verify accuracy.
- Fly levelling = reconnaissance (preliminary, rough; done before main levelling).
- Check levelling = verification (done after main levelling to confirm results).
- Fly levelling is fast — long sights, no repeat readings, minimum change points.