Datum Surface - the common surface to which all height differences between points are related back to:

Australian Height datum (AHD) - Uses Mean Sea Level (MSL) determined from a number of tide gauges around the country.

Local Datum - An assumed height adopted for a point in a small localised area - usually used when:
(a) Connection to the AHD is difficult or unnecessary.
(b) Wish to use a datum where negative RL's will not occur - Mine site.

Reduced Level (RL) - refers to the height of a point above the Datum Surface.

Bench Mark (BM) - A permanent survey mark having a reduced level (RL). These points are generally established by government organisations and information such as their location, RL and access can be obtained from the relevant department. In Western Australia - Geodetic Branch of the Department of Land Administration (DOLA).

Temporary Bench Mark (TBM) - a bench mark placed by a surveyor during the course of a survey for vertical height control. Generally no public records exist for these marks.

Level Surface - defined as a surface having a constant RL. On the Earth's surface it is in fact a curved surface.

Learn the terms

Surveying introduces many new terms and concepts and the levelling component is no different. Prior to exploring the following pages read and familiarise yourself with the terms.