Figure of the Earth
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In geodesy, the figure of the Earth is the size and shape used to model planet Earth. The kind of figure depends on application, including the precision needed for the model. A spherical Earth is a well-known historical approximation that is satisfactory for geography, astronomy and many other purposes. Several models with greater accuracy have been developed so that coordinate systems can serve the precise needs of navigation, surveying, cadastre, land use, and various other concerns.
A view across a 20-km-wide bay in the coast of Spain. The curvature of the Earth is evident in the horizon across the image, and the bases of the buildings on the far shore are below that horizon and hidden by the sea.
Geoid undulation in false color, shaded relief and vertical exaggeration (10000 scale factor).
Geoid undulation in false color, to scale.
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.