It is a vertical distance between survey point and adopted datum surface.
[1] Thus, it is considered as the base level which is used as reference to reckon heights or depths of other places or structures in that area, region or country.
[3] The most common and convenient datum which is internationally accepted is mean sea level which is a universal measure and based upon a common base line in the whole world determined by earth's gravitational model (see geoid) that gives the standard to measure elevation of a place above or below mean sea level.
National survey departments of each country determine RLs of significantly important locations or points.
The permanent benchmarks act as reference points for determining RLs of other locations in a particular country.