Datum
In geology, geography, cartography, surveying and other disciplines, a datum is a reference surface against which measurements can be made. Datum is also the singular for data.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, it should be remembered that sea-level has not stayed constant throughout geological time (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.
