Overview

A geodetic datum is a defined reference system that enables positions on or near the Earth's surface to be expressed as coordinates. Datums provide the origin, orientation and scale for coordinate systems used in mapping, surveying and navigation. When a latitude/longitude or grid value is given, the datum tells you which real-world location those numbers refer to.

Components and types

A datum normally has several linked components:

  • Reference ellipsoid — a mathematical surface approximating the shape of the Earth used to compute coordinates.
  • Geoid or vertical datum — a reference for measuring heights, often tied to mean sea level in a region.
  • Origin and orientation — the fixed point and alignment that anchor the datum to the planet.
  • Realization (reference frame) — the practical set of surveyed points and parameters that implement the datum.

Common distinctions are between horizontal datums (for latitude/longitude and planar coordinates) and vertical datums (for elevation). Datums may be local (optimized for a country or region) or geocentric/global (centered on the Earth's mass).

History and development

Datums evolved as surveying improved and global positioning systems became available. Historically, local datums were chosen to best fit the regional shape of the Earth. Later, geocentric global datums were developed to provide consistency worldwide and to support satellite navigation. Datums are periodically refined as measurement techniques improve and as tectonic motion is accounted for.

Uses, examples and importance

Datums are essential for producing accurate maps, integrating geospatial datasets, guiding construction or engineering projects, and operating satellite navigation systems. Examples of widely referenced datums include global frames used by GPS and regional datums used by national mapping agencies. If two datasets use different datums, their coordinates will not align unless transformed.

Practical issues and transformations

Switching between datums requires a defined transformation that may include translations, rotations, scale changes and sometimes time-dependent adjustments to account for plate motion. Surveyors and GIS practitioners must always note the datum when recording or using coordinates; otherwise a coordinate like (latitude, longitude) or a projected grid point may reference a different point on the ground. For map visualization and data exchange, specifying the datum avoids costly positioning errors.

Notable distinctions

Do not confuse a datum with a coordinate system: a datum supplies the reference; a coordinate system is the rule for assigning numbers. Also, the geoid (a physical equipotential surface approximating mean sea level) differs from the mathematical ellipsoid used by many horizontal datums. For further general information on mapping and location concepts see maps and mapping references.