2/28/2023 0 Comments Geodist databaseIt is essentially the basis for geodetic positioning by the Global Positioning System (GPS) and is thus also in widespread use outside the geodetic community. This system was adopted at the XVII General Assembly of the International Union of Geodesy and Geophysics ( IUGG). The 1980 Geodetic Reference System ( GRS 80) posited a 6,378,137 m semi-major axis and a 1:298.257 flattening. The relationship depends on the internal density distribution, or, in simplest terms, the degree of central concentration of mass. Its relationship with the geometrical flattening is indirect. The mechanical ellipticity of Earth (dynamical flattening, symbol J 2) can be determined to high precision by observation of satellite orbit perturbations. ![]() The quantity f = a − b / a, where b is the semi-minor axis (polar radius), is a purely geometrical one. It varies globally between ☑10 m, when referred to the GRS 80 ellipsoid.Ī reference ellipsoid, customarily chosen to be the same size (volume) as the geoid, is described by its semi-major axis (equatorial The geometrical separation between the geoid and the reference ellipsoid is called the geoidal undulation. The geoid, unlike the reference ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning. It is an idealized equilibrium surface of sea water, the mean sea level surface in the absence of currents and air pressure variations, and continued under the continental masses. The geoid is essentially the figure of Earth abstracted from its topographical features. Main articles: Geoid and Reference ellipsoid For this reason, the study of Earth's gravitational field is called physical geodesy. This applies to the solid surface, the liquid surface ( dynamic sea surface topography) and Earth's atmosphere. To a large extent, the shape of Earth is the result of rotation, which causes its equatorial bulge, and the competition of geological processes such as the collision of plates and of volcanism, resisted by Earth's gravitational field. It is also the science of measuring and understanding Earth's geometric shape, orientation in space, and gravitational field. Such geodetic operations are also applied to other astronomical bodies in the Solar System. Geodesy in the German-speaking world is divided into "higher geodesy" ( Erdmessung or höhere Geodäsie), which is concerned with measuring Earth on the global scale, and "practical geodesy" or "engineering geodesy" ( Ingenieurgeodäsie), which is concerned with measuring specific parts or regions of Earth, and which includes surveying. ![]() It is primarily concerned with positioning within the temporally varying gravitational field. The word geodesy comes from the Ancient Greek word γεωδαισία geodaisia (literally, "division of Earth"). ![]() 13.2 20th century geodesists (alphabetically arranged).13.1 Geodesists before 1900 (arranged by date).
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