On Wed, 4 Sep 2002, Nate Duehr wrote:

>My understanding of it is that the GPS system gives an altitude above a
>"fake" globe, which may or may not exactly match the "real" globe, which
>isn't perfect and has bumps and bulges... (GRIN)...

The fake globe is called a geoid and there are settings on GPS units to
switch that. The short answer is, that if the local altitude on your map
and the GPS current setting for reference geoid differ, you may misread in
altitude, potentially a lot (certainly enough to hit a tall point in a
plane in thick fog or to have tunnels dug into a hill at wrong heights on
the two sides, or to build a water pipe that will go uphill without your
knowing and other lovely things like that). The reference geoid (aka
survey date etc) is usually marked on the edge of the map near where the
scale is. If it isn't, worry.

Peter

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