Spectra Precision ProMark 800 Reference Manual - Manual (Page 76)

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Precise Surveying - Field Applications & Concepts
The reference ellipsoid refers to the WGS84, or better the
ITRFxx, where xx is the year the ITRF was realized (e.g.
ITRF00 was realized in the year 2000).
The center of this reference ellipsoid coincides with the
center of the mass of the Earth, which is also the origin point
of the Earth-Centered Earth-Fixed (ECEF) X, Y, Z Cartesian
coordinate system.
As far as horizontal coordinates are concerned, the reference
ellipsoid gives full satisfaction. Converting geographic
coordinates to any projection system does not raise any
particular problem.
Things are a bit more complicated though when dealing with
vertical coordinates because surveyors need to define very
precisely which type of vertical coordinates they wish to
measure.
The vertical coordinate provided by GNSS systems basically
is the height of the surveyed point over the reference
ellipsoid. We call it the ellipsoidal height.
For a long time, surveyors have used the concept of “mean
sea level” to measure the elevations of their points. The mean
sea level was the common “zero” elevation. But this concept
has shown some limitations.
Today, a much better model of vertical reference system,
called geoid, is used. This model is defined as a surface on
which the pull of gravity is constant.
Ellipsoidal Height
Reference Ellipsoid
Surveyed Point