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    GPS Position Accuracy (fwd)
    From: J. Florey
    Date: 1998 Jan 30, 9:09 AM

    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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