Datum
Overview
Datum is a reference system used to define the position of geographic features on the Earth’s surface, often used in conjunction with coordinate systems and projections in GIS. A geodetic datum defines the size and shape of the Earth (ellipsoid) and the origin and orientation of coordinate axes.
Key Concepts
Ellipsoid is the mathematical model of Earth’s shape. Reference frame is the physical realization of a datum through survey monuments. Horizontal datum defines positions on Earth’s surface (latitude/longitude). Vertical datum defines elevation reference (mean sea level). Datum shift is the transformation between different datums.
Common Horizontal Datums
| Datum | Ellipsoid | Use |
|---|---|---|
| WGS 84 | WGS 84 | GPS, global |
| NAD 83 | GRS 80 | North America |
| NAD 27 | Clarke 1866 | Legacy US |
| ETRS 89 | GRS 80 | Europe |
Vertical Datums
- NAVD 88 (North American Vertical Datum 1988)
- NGVD 29 (National Geodetic Vertical Datum 1929)
- EGM96/EGM2008 (Earth Gravitational Model)
Appendix
Created: 2025-12-13 | Modified: 2025-12-13