a = semi-major (equatorial) axis
b = semi-minor (polar) axis
Flattening: f = 1 – b/a (often used as 1/f)
Going back to the early 19th century, only about 20 ellipsoids have been used for mapping. Legal and practical issues--all maps and legal property descriptions are based on the datum--preclude changing the ellipsoid and datum every time we obtain a slightly better estimate for the earth's shape.
|Table. Commonly Used Ellipsoids. (DMA, 1997, Appendix A.1)|
Last revision 11/13/2017