- datum
- the geometric relationship between a coordinate system grid and coordinates on the Earth’s surface
- ellipsoid
- idealized ellipsoidal geometry representing a best-fit approximation of the geoid (roughly mean sea level) (e.g. GRS80)
- ensemble
- for datums like WGS that have many realizations, the ‘ensemble’ is the collection of all realizations. Data collected without specifying a realization consequently is labeled as referenced to the ensemble and has higher intrinsic uncertainty
- epoch
- the timestamp of data collection
- frame epoch
- date for which a set of coordinates within a datum are considered valid
- geoid
- best-fit irregular equipotential surface that closely approximates global mean sea level
- orthometric height
- Local elevation measured normal to geoid surface (rather than ellipsoid surface)
- realization
- a specific datum definition consiting of origin, orientation, and scale of the coordinate system, usually determined empirically at a specific point in time