In the context of levelling, a level surface is defined as a:
Correct Answer: C. curved surface
📚 Detailed Explanation: A Level Surface Is a Curved Surface
Why C (curved surface) is correct: A level surface is not flat — it curves around the Earth following the mean spheroidal shape. Every point on a level surface is at the same gravitational potential and lies perpendicular to the local direction of gravity (the plumb line).
Types of Surveying Surfaces
| Surface Type | Definition | Shape |
|---|---|---|
| Level surface | An equipotential surface of gravity; concentric with the mean spheroid of the Earth; every point perpendicular to the local plumb line | Curved (follows Earth’s curvature) |
| Horizontal surface | A flat plane tangent to the level surface at a single point; departs from the level surface as distance increases | Flat (plane) |
| Vertical surface | A surface containing the plumb line; perpendicular to the level surface | Plane (or curved radial) |
| Datum surface | The reference surface from which elevations are measured (usually MSL); itself a level surface | Curved |
Why the Distinction Matters in Levelling
Because level surfaces are curved, a truly horizontal line of sight departs ABOVE the level surface:
Departure at d km = 0.0785 × d² metres (curvature correction)
Refraction partially corrects this:
Net combined correction = 0.0673 × d² metres
For short distances (<100 m) the departure is negligible; for long distances it must be corrected.
Departure at d km = 0.0785 × d² metres (curvature correction)
Refraction partially corrects this:
Net combined correction = 0.0673 × d² metres
For short distances (<100 m) the departure is negligible; for long distances it must be corrected.
- Level surface = curved, concentric with the mean spheroid of the Earth.
- Horizontal surface = flat plane tangent to the level surface at one point only.
- The difference between a level surface and a horizontal plane increases with distance from the point of tangency.