## Geodesy
Geodesy is the science concerned with the exact size and shape of the surface
of the Earth. It also involves the study of variations of the Earth's gravity.
## The Shape of the Earth
The Greek philosophers were the first to theorize that the Earth was round.
However, in their speculation and theorizing, the shape of the Earth ranged
from the flat disc advocated by Homer to Pythagoras' spherical figure -
an idea supported one hundred years later by Aristotles. Pythagoras was
a mathematician and to him the most perfect figure was a sphere. He reasoned
that the gods would create a perfect figure and therefore the Earth was
created to be spherical in shape. Anaximenes, an early Greek scientist,
believed strongly that the Earth was rectangular in shape.
Ancient Greek philosophers concluded that the Earth could only be a
sphere because that, in their opinion, was the "most perfect" shape. Today
we know that the shape of the Earth is much more complex and consists of
a very complex and irregular topographic surface. ## Geodetic Systems
Since the Earth is in fact flattened slightly at the poles and bulges somewhat
at the equator, the geometrical figure used in geodesy to most nearly approximate
the shape of the Earth is an ellipsoid of revolution. The ellipsoid of
revolution is the figure which would be obtained by rotating an ellipse
about its shorter axis.
At the 1967 meeting of the IUGG held in Lucerne, Switzerland, the ellipsoid
called GRS-67 in the listing was recommended for adoption. The new ellipsoid
was not recommended to replace the International Ellipsoid (1924), but
was advocated for use where a greater degree of accuracy is required. It
became a part of the Geodetic Reference System 1967 which was approved
and adopted at the 1971 meeting of the IUGG held in Moscow. It is used
in Australia for the Australian Geodetic Datum and in South America for
the South American Datum 1969.
## Global Reference Systems and Reference Frames
An important underlying concept is that definitions of reference systems
are purely definitions and must be "realised" through some defined process.
At the most fundamental level, two types of reference systems are of interest.
The first is the Celestial Reference System (CRS) which is a space fixed
system to which the positions of celestial objects are referred. The origin is at the centre of mass of the whole Earth including the oceans and atmosphere. The unit of length is the metre. The orientation of its axes is consistent with that of the Bureau International de l'Heure (noe IERS) at the beginning of 1984. Changes in orientation over time are such that there is no residual rotation with respect to the horizontal movement of the Earth's crust.
The International Earth Rotation Service (IERS)
has been established since 1988 jointly by the International Astronomical
Union (IAU) and the International Union
of Geodesy and Geophysics (IUGG). The
IERS mission is to provide to the worldwide scientific and technical community
reference values for Earth orientation parameters and reference realizations
of internationally accepted celestial and terrestrial reference systems. International Terrestrial Reference Frame (ITRF), WGS84 as used for GPS and PZ90 as used for GLONASS.
## WGS-84WGS-84 is an Earth fixed global reference frame, including an Earth model, which was established by the US Defense Mapping Agency (now the National Imaging and Mapping Agency, NIMA). It is defined by a set of primary and secondary parameters: the primary parameters define the shape of an Earth ellipsoid, its angular velocity, and the Earth mass which is included in the ellipsoid reference the secondary parameters define a detailed gravity model of the Earth.
These additional parameters are needed because WGS-84 is used not only
for defining coordinates in surveying, but, for example, also for determining
the orbits of GPS navigation satellites. Working with WGS-84
It should be noted that there are only two ways to directly produce
WGS-84 coordinates. The first is by GPS
surveying measurements relative to the US DoDs GPS tracking stations.
However, the GPS data from those DoD
stations is not typically available to civilians. The second way is
by point positioning using a GPS receiver.
However, the accuracy of point positions performed by civilians is
limited by the policy of Selective Availability to
+/- 100m at 95% confidence. Only US DoD or allied military agencies
can perform point positioning with
centimetre to decimetre accuracy. |

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