A geodetic datum (plural is datums) definition has a change of semantics and meaning.
modern geodetic reference systems
are determinded in a combination of many geodetic and
astronomic measurements like Global
Satelite Navigation System (GNSS), Very Long
Baseline Interferometry (VBLI) etc. They are positionned earth centered.
- at the beginning, a geodetic datum
was the astronomic and geodetic determination of size,
position (fundamental point) and orientation
(Laplace azimuth) of a local, regional
reference system. Mainly, they had no relationship to other reference systems
- in geomatics a geodetic datum often
means the datum transformation parameters from
a national to a global geodetic reference system.