Spectra Precision ProMark 800 Reference Manual - Manual (Page 77)

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Precise Surveying - Field Applications & Concepts
This surface is irregular depending on the density and
distribution of materials on the surface of the Earth, which
means the geoid may not exactly follow the natural features
on the Earth’s surface. (The geoid is a fictitious surface that
can’t be seen).
Sorry for being now a bit sarcastic, but using the geoid as
vertical reference, one can be sure water will always flow
downhill, from lower to higher gravity level, which was not
always the case when using the too-approximative mean sea
level!!
So the question is now, “How can we convert an ellipsoidal
height provided by our GNSS system into an elevation?”
In practical terms, a geoid model used in a GNSS system is
a file containing a more or less dense array of points evenly
distributed across the surface of the geoid. For each point,
the file provides the horizontal geographic coordinates and
the separation (geoidal separation) between the reference
ellipsoid and the geoid. The extent of the geoid file may be
worldwide or limited to a particular area.
Providing an accurate modelling of the undulations of the
geoid surface, the geoid file is used by the GNSS system to
interpolate the separation between this surface and the
surface of the reference ellipsoid for the point surveyed.
Reference Ellipsoid
Surveyed Point
Geoid
Elevation