A level line in surveying is:
Correct Answer: A. Parallel to mean spheroid
📚 Detailed Explanation: Level Line Is Parallel to Mean Spheroid (Repetition from Q64)
Why A (Parallel to mean spheroid) is correct: This question repeats the core concept from Q64. A level line is a line lying within a level surface (equipotential surface of gravity). It is everywhere perpendicular to the local direction of gravity and therefore runs parallel to (equidistant from) the mean spheroidal surface of the Earth.
Why a Level Line Curves with the Earth
| Concept | Explanation |
|---|---|
| Level surface | A curved surface where gravity potential is constant; it bulges slightly outward (away from the centre of the Earth) at the equator |
| Level line | A line lying in a level surface; it curves with the Earth and is parallel to the mean spheroidal surface at every point along its length |
| Horizontal line | A straight line tangent to the level surface at one point; it rises above the level surface with distance (diverges from the spheroid) |
| Curvature correction | The vertical divergence between a level line and a horizontal line = 0.0785D² m (D in km) |
- Level line = curved; parallel to mean spheroidal surface.
- Horizontal line = straight; tangent to level surface at instrument; diverges from it with distance.
- The difference between them is the curvature correction: Cc = 0.0785 D² m.