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    Re: Electronic vs non-electronic
    From: John Simmonds
    Date: 1999 Oct 04, 03:05 EDT

    One thing you've got to remember (at least in my part of the world) is
    that many charts are still base on surveysdone in the late 17th/early
    18th centuries. Remarkably accurate given the timepieces and other nav
    instruments in use then
    > Craig wrote:
    > OK, So which one is the problem?  The GPS or the charts?  If you are
    > actually at a location and the GPS does not match the chart, would
    > celestial match the chart or is the chart wrong?
    > Craig
    >      -----Original Message-----
    >      From: Navigation Mailing List
    >      [mailto:NAVIGATION-L@XXX.XXX]On Behalf Of
    >      Trayfors, William
    >      Sent: Friday, October 01, 1999 16:52
    >      Subject: Re: Electronic vs non-electronic
    >      Bob:
    >      You betcha.   For example, see my letter in Ocean Navigator
    >      May/June 1997 (p27) re: loss of a 70' custom sloop.  There
    >      are lots of other examples.
    >      I recently attended a conference in Hawaii at which one of
    >      the speakers presented a detailed and learned discussion on
    >      GPS errors and modern navigation/charting.  He used the
    >      term, "GPS-assisted collisions", to point out several
    >      problems.  One of these is that chart datums often differ
    >      considerably from the WGS84 GPS standard datum (in my
    >      Caribbean example there was a .2 NM N/S difference and a
    >      slightly smaller E/W difference).  The speaker gave an
    >      example in the South Pacific where a charted airstrip is
    >      actually 2km off the GPS position!  Islands are often
    >      mis-charted as well.
    >      There are several problems associated with making GPS
    >      positions jibe with charted positions, including:
    >      -  GPS system errors
    >      -  datum errors
    >      -  charted position errors
    >      - elipsoid (theoretical sphere on which GPS is based) vs.
    >      geoid (actual surface of the earth) differences
    >      etc.
    >      My bottom line is:  GPS is a wonderful tool, but not one to
    >      use blindly.  In unfamiliar waters especially, use it as you
    >      would a sextant, i.e., assume there could be a very sizeable
    >      error.
    >      Bill
    >      At 02:14 PM 10/1/99 -0400, you wrote:
    >      >Are there any horror stories out there involving navigation
    >      by GPS instead
    >      >of real navigation? Looking for situations where
    >      traditional navigation
    >      >would probably have saved the day when GPS fouled up.
    >      >
    >      >Bob
    >      __________________________________
    >      Bill Trayfors <btrayfors@XXX.XXX>
    >      The Washington Decision Support Group, Inc.
    >      Specialists in Advanced Information & Communications
    >      Technologies
    >      2401 South Lynn Street, Arlington, VA 22202
    >      Office (703) 838-8784   Tech Support (703) 573-WDSG   FAX
    >      (703) 838-0019

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