[geodesy]
A reference ellipsoid used as the basis for the North American Datum of 1927 (NAD27) and other datums, as well as for latitude and longitude on topographic and other maps produced in Canada, Mexico, and the United States from the late 1800s to the late 1970s. Largely replaced by the Geodetic Reference System 1980 (GRS80) or the World Geodetic System 1984 (WGS84). The Clarke ellipsoid of 1866 is also known as the Clarke spheroid of 1866.