**Spheroid, Ellipsoid,
and Geoid**

* Spheroid* is a solid generated by
rotating an ellipse about either the major or minor axis

* Ellipsoid* is a solid for which all
plane sections through one axis are ellipses and through the
other are ellipses or circles. If any two of the three axes of
that ellipsoid are equal, the figure becomes a spheroid
(ellipsoid of revolution). If all three are equal, it becomes a
sphere.

* Geoid* is the equipotential gravity
surface of the earth at mean sea level. At any point it is
perpendicular to the direction of gravity.

Reference:

Smith, James R., Introduction to Geodesy: The history and
concepts of modern geodesy. John Wiley & Sons, 1997.

*viljoen@gis.nrcan.gc.ca** 20/05/98*