The Hayford ellipsoid is a geodetic

Geodesy

Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...

 reference ellipsoid

Reference ellipsoid

In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body....

, named after the US geodesist John Fillmore Hayford

John Fillmore Hayford

- References :...

 (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

International Union of Geodesy and Geophysics

The International Union of Geodesy and Geophysics is a non-governmental organisation dedicated to the scientific study of the Earth using geophysical and geodesic techniques. The IUGG was established in 1919. Some areas within its scope are environmental preservation, reduction of the effects of...

 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

Semi-major axis

The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 a = 6,378,388.000 and its flattening

Flattening

The flattening, ellipticity, or oblateness of an oblate spheroid is a measure of the "squashing" of the spheroid's pole, towards its equator...

 f = 1:297.00. Unlike its predecessors, such as the Bessel ellipsoid

Bessel ellipsoid

The Bessel ellipsoid is an important reference ellipsoid of geodesy. It is currently used by several countries for their national geodetic surveys, in Europe and on other continents, but will be replaced in the next decades by modern ellipsoids of satellite geodesy.The Bessel ellipsoid was derived...

 (a = 6,377,397 m, f = 1:299,15), which was a European ellipsoid, the Hayford ellipsoid also included measurements from North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, as well as other continents (to a lesser extent). It also included isostatic measurements to reduce plumbline

Plumbline

A plumbline is a string with a lead weight or plumb-bob, used to provide a vertical reference line.It may also refer to:*The Plumbline, a joke newspaper produced by the McMaster Engineering Society...

 divergences.

It has since been replaced as the "International ellipsoid" by the newer Lucerne ellipsoid (1967) and GRS 80

GRS 80

GRS 80, or Geodetic Reference System 1980, is a geodetic reference system consisting of a global reference ellipsoid and a gravity field model.-Geodesy:...

 (1980).

Sources

• Defense Mapping Agency: Geodesy for the layman, 1983, p. 8 http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/geo4lay.pdf