Which of the following correctly defines a level line?
Correct Answer: A. Parallel to mean spheroidal surface
📚 Detailed Explanation: Level Line Is Parallel to Mean Spheroidal Surface
Why A (Parallel to mean spheroidal surface) is correct: A level line is a curved line lying in a level surface. A level surface is an equipotential surface of gravity — i.e., a surface everywhere perpendicular to the direction of gravity. The reference level surface is the mean spheroidal surface (mean geoid / MSL). Therefore, all level lines are parallel (equidistant) from the mean spheroidal surface.
Level Line vs. Horizontal Line vs. Datum
| Line / Surface | Shape | Relationship to Gravity | Relationship to Mean Spheroid |
|---|---|---|---|
| Level line | Curved (follows Earth) | Perpendicular to local gravity everywhere | Parallel to (equidistant from) mean spheroid |
| Horizontal line | Straight (tangent to level line at one point) | Perpendicular to gravity at one point only | Diverges from spheroid with distance |
| Vertical line | Straight (along gravity direction) | Parallel to gravity (coincides with plumb line) | Cuts spheroid at right angles |
| Mean spheroidal surface (datum) | Spheroid (slightly flattened sphere) | Everywhere perpendicular to mean gravity | IS the reference level surface |
- Level line = curved line in a level surface; parallel to the mean spheroidal surface.
- Over short distances, level line ≈ horizontal line; over long distances they diverge significantly.
- The vertical datum (MSL) is the particular level surface chosen as RL = 0.