Levelling MCQ with answer and solution

Concept

A level surface is a curved surface that is perpendicular to the plumb line (direction of gravity) at every point.

Step-by-step solution

1. A level surface is perpendicular to the direction of gravity (plumb line) at every point. 2. The earth is nearly spherical, so plumb lines at different points are not parallel. 3. So a level surface follows the earth's curvature and is parallel to the mean spheroidal surface of the earth. 4. Hence a level surface is a curved surface.

Explanation

A level surface is perpendicular to the plumb line at every point. Since the earth is curved, this surface is also curved, like the surface of still water in a lake. A horizontal surface is only a plane tangent to the level surface at one point.

Why the other options are wrong

A: A datum is one chosen level surface used as reference; not every level surface is a datum. B: A horizontal surface is a flat plane tangent to the level surface at one point only. C: A vertical surface contains the plumb line, so it is not level.

Shortcut

Level = curved (follows the earth); horizontal = flat (tangent plane).

Common mistake

Thinking level surface and horizontal surface are the same thing.