This article describes a concept from cartography and geodesy. Datum is also the singular form of data.

In cartography, surveying, geodesy and other disciplines, a datum is a reference point or surfact against which position measurements are made.

For instance, sea-level is commonly used as a datum against which height (or depth) of topographic features can be measured. Whilst the use of sea-level as a datum is useful for geologically recent topographic features, sea-level has not stayed constant throughout geological time, so is less useful when measuring very long-term processes (see Vail-curve).

On maps, heights and depths will typically be given in height above datum or depth below datum. Ordnance Survey maps define the Ordnance Survey Datum or OSD which is usually sea-level, but should be checked in the map legend.

On nautical charts, the depth and height of features shown on a chart are measured relative to the level of the "lowest astronomical tide", that is the lowest tide caused by gravity alone with no contribution from the weather.

Positions on nautical charts are also relative to a horizontal map datum such as WGS84.

External links

• UK Ordnance Survey (http://www.ordsvy.gov.uk/)
• US National Geodetic Survey (http://www.ngs.noaa.gov/)