the geometric relationship between a coordinate system grid and coordinates on the Earth’s surface
ellipsoid
idealized ellipsoidal geometry representing a best-fit surface through the Earth’s topography (e.g. GRS90)
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