Datums

A datum is a reference point, line, or surface used as the basis for measuring and defining other quantities. The term is applied across disciplines to provide a standard against which coordinates, elevations, or data values are measured or compared.

In surveying and mapping, a geodetic datum defines the size and shape of the reference model of the Earth (an ellipsoid) and the origin or orientation for a coordinate system. Horizontal datums establish geographic coordinates (latitude and longitude) relative to the ellipsoid, while vertical datums provide elevations relative to a reference surface, such as mean sea level. Global datums, like the World Geodetic System 1984 (WGS84), aim for worldwide consistency, whereas local datums (NAD83 for North America, ETRS89 for Europe, NAD27 in older North American maps) optimize accuracy for a region. Datums are linked to a geoid model and transformations between datums account for shifts and rotations.

In data management, datum refers to a single piece of information or a single observation within a dataset; it is the singular form of data. In everyday usage, data is often treated as a mass noun, but technically a datum is one value or record.

Vertical datums anchor elevations to a reference surface such as mean sea level, while temporal datums attach time values to a standard epoch. The choice of datum influences measurement accuracy, compatibility, and interoperability among geographic information systems and datasets.