5.1c: When is it NOT okay to assume the Earth is an ellipsoid?
Description
This article is from the Geographic Information Systems FAQ, by Lisa Nyman lnyman@census.gov with numerous contributions by others.
5.1c: When is it NOT okay to assume the Earth is an ellipsoid?
The shape the Earth would assume if it were all measured at mean sea level
is called the geoid. The geoid varies no more than about a hundred meters
above or below a well-fitting ellipsoid, a variation far less than the
ellipsoid varies from the sphere. Terrain relief is reported relative to
the geoid. (Paraphrased from p. 11 of the book by Snyder cited above.)
Distances on the surface of the geoid are not particularly meaningful.
However, there are applications, such as long-term prediction of orbits of
Earth satellites, that require better approximations than are provided by
an ellipsoid. Astrodynamics texts, such as
Kaula, William M. 1966 _Theory of Satellite Geodesy_ Blaisdell
Publishing Co., Waltham, MA (This book may be out of print.)
Battin, Richard H. 1964 _Astronautical Guidance_ McGraw-Hill,
New York (There may be later editions.)
may be consulted for further information.
Distances on the surface of the terrain, whether geodesic, on roads,
cross-country, or straight-line, depend on relief (including elevation
differences), the status of engineering projects, and perhaps even route
selection. Hence, computation is idiosyncratic and not well suited to
simple approximations.
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