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Grade measurement is the geodetic determination of the local radius of curvature of the figure of the Earth by determining the difference in astronomical latitude between two locations on the same meridian, the metric distance between which is known.
The first known grade measurement was performed by Eratosthenes (240 BC) between Alexandria and Syene in what is now Egypt, determining the radius of the Earth with remarkable correctness. The Dutch geodesist Snellius (~1620) repeated the experiment between Alkmaar and Bergen op Zoom using more modern geodetic instrumentation.
Later grade measurements aimed at determining the flattening of the Earth ellipsoid by measuring at different geographic latitudes. The first of these was the one commissioned by the French Academy of Sciences in 1735-1738, involving measurement expeditions to Lapland (Maupertuis et al.) and Peru (Pierre Bouguer et al.).
Later, Struve measured a geodetic triangle chain between the Arctic Sea and the Black Sea, and Bessel compiled several meridian arcs to compute the famous Bessel ellipsoid (1841). Nowadays, the method is replaced by worldwide geodetic networks and by satellite geodesy.
Also the imaginary grade measurement described by Jules Verne in his book "Adventures of 3 Russians and 3 Englishmen in South Africa" of 1872 may be mentioned.
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