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One more reason to keep alive the traditional arts.  Note the error in GPS
positions reported for certain parts of the world.

Regards,
Jay Florey

---------- Forwarded message ----------
Date: Tue, 27 Jan 1998 14:55:45 GMT
From- C J Brady 
Newsgroups: misc.transport.marine
Subject: GPS Position Accuracy


Subject: GPS position accuracy and EGPWS

The recent note on EGPWS prompted me to write this note.  I don't remember
having seen a discussion in RISKS about the GPS reference datum problem
that
can be a significant hazard.

In short, the GPS system uses a model of the Earths shape (an ellipsoid)
that is suitable for the entire Earth.  Almost all printed maps, and quite
likely many of the electronic maps derived from them, use an assortment of
ellipsoids that were typically optimized for a country or region.  The GPS
position when plotted on one of these maps can be off by a considerable
amount, as much as seven miles in some Pacific areas.  The half mile
discrepancy in the Caribbean has already led to several boat wrecks when
they tried to navigate a narrow passage in poor visibility using GPS data.

Considerable detail is available in yachting publications (e.g., the
current
Ocean Navigator magazine), but the EGPWS article shows a way in which
aviators might be using unsurveyed approaches that would be subject to
this
risk.

Ron



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