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Ellipsoidal Height and Elevation
50% of the points in the horizontal scatter plot (see chart
below). This means the computed position will be within
the announced accuracy 50% of the time.
The vertical coordinate measured by GNSS systems is worth
an explanation. Behind this coordinate in fact lies the
specificity of GNSS systems compared to conventional
surveying systems.
Basically, all positions delivered by GNSS systems consist of
geographic coordinates (latitude, longitude, height)
referenced to an ellipsoid, called reference ellipsoid, which is
a simple and accurate model to describe the shape and
surface of our planet.
The reference ellipsoid refers to the WGS84, or better the
ITRF00.
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.
Precise Surveying - Field Applications & Concepts
Y0
CEP
X0
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