Vertical datum

For the purpose of measuring the height of objects on land, the usual datum used is mean sea level (MSL).

This definition averages out tidal highs and lows (caused by the gravitational effects of the sun and the moon) and short term variations.

It will not remove the effects of local gravity strength, and so the height of MSL, relative to a geodetic datum, will vary around the world, and even around one country.

Conversely, if a ship is to safely pass under a low bridge or overhead power cable, the mariner must know the minimum clearance between the masthead and the obstruction, which will occur at high tide.

Sea level does not remain constant throughout geological time, and so tidal datums are less useful when studying very long-term processes.

In some situations sea level does not apply at all – for instance for mapping Mars' surface – forcing the use of a different "zero elevation", such as mean radius.

[8] Common chart datums are lowest astronomical tide (LAT)[8] and mean lower low water (MLLW).