The line normal (perpendicular) to the plumb line is called:
Correct Answer: A. Level line
📚 Detailed Explanation: The Line Normal to the Plumb Line Is Called a Level Line
Why A (Level line) is correct: A level line is defined as a line that lies within a level surface and is therefore everywhere perpendicular (normal) to the local direction of gravity (the plumb line). At any given point, the level line is perpendicular to the plumb line passing through that point.
Key Terms in Levelling Theory
| Term | Definition | Relationship to Plumb Line |
|---|---|---|
| Level line | A line lying in a level surface; curves with the Earth; connects all points of equal gravitational potential | Perpendicular (normal) to plumb line at every point |
| Horizontal line | A straight line tangent to the level surface at one point; departs from the level surface with distance | Perpendicular to plumb line only at the point of tangency |
| Vertical line | A line coinciding with the plumb line; direction of gravity | Parallel to the plumb line |
| Datum line | The reference line or surface (e.g., MSL) from which elevations are measured | Itself a level surface; perpendicular to plumb line everywhere |
Key Distinction: A horizontal line is a straight line perpendicular to the plumb line at one point. A level line is a curved line perpendicular to the plumb line at every point along its length. Over short distances they are nearly identical; over long distances they diverge significantly.
- Level line = perpendicular to the local plumb line (gravity direction) at every point.
- Level line = curved (follows the mean spheroid); horizontal line = flat (tangent at one point).
- The line normal (perpendicular) to the plumb line is the level line.