- Grade measurement
Grade measurement is the geodetic determination of the local radius of curvature of the
figure of the Earth by determining the difference in astronomicallatitude 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 geodesistSnellius [~1620) repeated the experiment betweenAlkmaar andBergen op Zoom using more modern geodetic instrumentation.Later grade measurements aimed at determining the
flattening of the Earth ellipsoid by measuring at differentgeographic latitude s. 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, andBessel compiled several arcs to compute the famousBessel ellipsoid (1841). Nowadays, the method is replaced by worldwidegeodetic network s and bysartellite 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."See also:"
*Erdmessung ,Earth ellipsoid
*Meridian arc ,Astrogeodesy
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