Following up on my ealier post regarding GPS alitmeters vs barometric altimeters. I'm a bit of a map geek, and to some extent use this stuff in my work, but the whole concept of geoids, spheroids and datums gets very confusing to many people. If you want a little more understanding of these subjects, I can recommend two sources:

The book How To Read A Nautical Chart by Nigel Calder (2003, International Marine) has by far the clearest discussion I have seen for explaining the geoid, spheroids and datums, and why they matter. The book is aimed at sailors, but the first couple of chapters are useful background for anyone using maps and charts of any kind.

Another source, which is the book Geodesy For The Layman (1984) which is available online from NOAA at
http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003A.HTM
and in pdf at
http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/geo4lay.pdf
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz