The International Union of Geodesy and Geophysics is an international non-governmental organization dedicated to the scientific study of Earth and its space environment using geophysical and geodetic techniques.
A German stamp about the IUGG General Assembly in 1983, Hamburg
Geodesy
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Geodesy is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems.
A modern instrument for geodetic measurements using satellites
Geoid, an approximation for the shape of the Earth; shown here with vertical exaggeration (10000 vertical scaling factor).
Height measurement using satellite altimetry
GPS Block IIA satellite orbits over the Earth.