Ordnance datum

Ordnance Datum is a specific reference point used in surveying and mapping that provides a standard level from which height or elevation measurements can be made. In the context of the United Kingdom, the term commonly refers to the mean sea level at Newlyn, Cornwall, which has been used as a base for height measurements since the 19th century. Measurements taken relative to Ordnance Datum are typically expressed in meters or feet above this reference level.