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  1. 1. Hayford, John Fillmore Born May 19, 1868, in Rouses Point, N.Y.; died Mar. 10, 1925, in Evanston, 111. American geo-desist. From 1906 to 1909, using the theory of isostasy, Hayford computed the dimensions of the earth ellipsoid based on arc measurements covering the USA. Although the figure Hayford deduced was adopted in 1924 as the International Ellipsoid, it has not been widely used. From studies of isostasy and gravity anomalies, Hayford estimated the depth of isostatic compensation to vary from 60 to 122 km (37 to 76 miles) and from that deduced the figure of the Earth, which was adopted in 1924 as the International Ellipsoid by the International Geodetic and Geophysical Union. Hayford wrote Geodetic Astronomy (1898). He served as a member of the U.S. Coast and Geodetic Survey periodically from 1889 until 1909, when he became director of the College of Engineering at Northwestern University, Evanston, Ill. The Hayford ellipsoid is a geodetic reference ellipsoid, named after the US geodesist John Fillmore Hayford (1868-1925), which was introduced in 1910. The Hayford ellipsoid was also referred to as the International ellipsoid 1924 after it had been adopted by the International Union of Geodesy and Geophysics IUGG in 1924, and was recommended for use all over the world. Many countries retained their previous ellipsoids. The Hayford ellipsoid is defined by its semi-major axis a = 6,378,388.000 and its flattening f = 1:297.00. Unlike some of its predecessors, such as the Bessel ellipsoid (a = 6,377,397 m, f = 1:299,15), which was a European ellipsoid, the Hayford ellipsoid also included measurements from North America, as well as other continents (to a lesser extent). It also included isostatic measurements to reduce plumbline divergences. Hayfords ellipsoid did not reach the accuracy of Helmerts ellipsoid published 1906 (a = 6378 200 m ; Erdabplattung f = 1 : 298.3). It has since been replaced as the "International ellipsoid" by the newer Lucerne ellipsoid (1967) and GRS 80 (1980).