Arc measurement
Arc measurement,[1] sometimes degree measurement[2] (German: Gradmessung, after the gradian),[3] is the astrogeodetic determination of the local radius of curvature of the figure of the Earth by determining the difference in astronomical latitude and longitude between two locations, the arc length between which is known. The most common instantes, meridian arc measurements, involved only astronomical latitude and the meridian arc length, although longitude (parallel) and oblique arc measurements are also possible.[1]
This article may be expanded with text translated from the corresponding article in German. (May 2021) Click [show] for important translation instructions.

The first known arc 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 (Snellius' triangulation).
Later arc measurements aimed at determining the flattening of the Earth ellipsoid by measuring at different geographic latitudes. The first of these was the French Geodesic Mission, commissioned by the French Academy of Sciences in 1735–1738, involving measurement expeditions to Lapland (Maupertuis et al.) and Peru (Pierre Bouguer et al.).
Struve measured a geodetic control network via triangulation between the Arctic Sea and the Black Sea, the Struve Geodetic Arc. 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.
Other instances
 AlMa'mun's arc measurement
 Posidonius' arc measurement
 Swedish–Russian ArcofMeridian Expedition
 Picard's arc measurement
 DunkirkCollioure arc measurement (Cassini, Cassini, and de La Hire)
 DunkirkCollioure arc measurement (Cassini de Thury and de Lacaille)
 Meridian arc of Delambre and Méchain
 West EuropeAfrica Meridianarc
 De Lacaille's arc measurement
 Fernel's arc measurement
 Norwood's arc measurement
 Boscovich and Maire's arc measurement
 Maclear's arc measurement
 Hopfner's arc measurement
See also
References
 Torge, W.; Müller, J. (2012). Geodesy. De Gruyter Textbook. De Gruyter. p. 5. ISBN 9783110250008. Retrieved 20210502.
 Jordan, W., & Eggert, O. (1962). Jordan's Handbook of Geodesy, Vol. 1. Zenodo. http://doi.org/10.5281/zenodo.35314
 Torge, W. (2008). Geodäsie. De Gruyter Lehrbuch (in German). De Gruyter. p. 5. ISBN 9783110198171. Retrieved 20210502.