The height of any point with respect to mean sea level (MSL) is called:
Correct Answer: D. Reduced level
📚 Detailed Explanation: Height of Any Point w.r.t. MSL = Reduced Level (RL)
Why D (Reduced level) is correct: The Reduced Level (RL) of a point is its vertical elevation measured above (or below) a reference datum — usually Mean Sea Level (MSL). It is the fundamental quantity computed in every levelling survey.
Key Levelling Terminology
| Term | Definition |
|---|---|
| Bench Mark (BM) | A permanent reference point of known RL; the starting point for a levelling survey |
| Datum | The reference surface (usually MSL) from which RLs are measured; not a point elevation |
| Level surface | A curved equipotential surface at the same gravitational potential; not the same as a single point elevation |
| Reduced Level (RL) | The vertical height of a specific point above (or below) the adopted datum (MSL); expressed in metres |
Example:
Point A: RL = 105.34 m → A is 105.34 m above MSL
Point B: RL = -2.50 m → B is 2.50 m below MSL
Point C: RL = 0.000 m → C is exactly at MSL
Point A: RL = 105.34 m → A is 105.34 m above MSL
Point B: RL = -2.50 m → B is 2.50 m below MSL
Point C: RL = 0.000 m → C is exactly at MSL
- Reduced Level (RL) = vertical elevation of a point above (or below) the datum (MSL).
- RL is the quantity that levelling surveys compute for each station.
- Bench Mark is a point of known RL; it is not the definition of “height above MSL” itself.