[geodesy]
A reference ellipsoid having a semimajor axis of approximately 6,378,206.4 meters and a flattening of 1/294.9786982. It is the basis for the North American Datum of 1927 (NAD27) and other datums. Used as the basis 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. The Clarke ellipsoid of 1866 is also known as the Clarke spheroid of 1866.
