What does levelling involve?
- Measuring vertical distances with reference to the sea level
- Measuring vertical distances with reference to the equator
- Measuring vertical distances with reference to a vertical plane
- Measuring vertical distances with reference to a horizontal plane
- Remember the definition of levelling?
- Think about the levelling process as discussed.
Since levelling is finding out the elevation of a point with respect to a datum, what is used as a vertical datum? The aswer is a horizontal plane. The measurement of elevation is done along a vertical (imaginary) line, which is orthogonal to a horizontal plane.
If you are confused about the option that mentions the "sea level", note that it does not refer to the Mean Sea Level (notice the difference when we include the word "mean"). The Mean Sea Level is indeed a type of vertical datum, but that is not what is mentioned.