The surface of still (undisturbed) water is an example of:
Correct Answer: A. Level surface
📚 Detailed Explanation: Still Water Surface Is a Level Surface
Why A (Level surface) is correct: A still (undisturbed) water surface naturally assumes the shape of a level (equipotential) surface, because water flows until the gravitational potential is equal everywhere on its surface. Therefore, the surface of still water is, by definition, a level surface.
Level Surface Properties
| Property | Description |
|---|---|
| Definition | A curved surface where the gravitational potential is constant at every point |
| Relationship to gravity | Perpendicular to the direction of gravity (plumb line) at every point |
| Physical example | Surface of still water — water flows until equal potential everywhere; then stops |
| Shape | Curved (follows the spheroidal shape of the Earth); not a flat plane |
| Relationship to MSL | The geoid (MSL surface) is one specific level surface; all other level surfaces are parallel to it |
Why not a horizontal surface? A horizontal surface is a flat plane. The surface of still water is curved (it follows the Earth's curvature), making it a level surface, not a horizontal plane. Over short distances they are nearly identical, but over longer distances the difference is the curvature correction (0.0785D² m).
- Still water surface = level surface (water distributes itself along equal gravitational potential).
- Level surface is curved; horizontal surface is flat — they differ by the curvature correction.
- The geoid (datum of levelling) is modelled on the still water surface at mean sea level.