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    Re: Electronic vs non-electronic
    From: Craig Scott
    Date: 1999 Oct 03, 19:24 EDT

    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?
      -----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
      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
      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.
      At 02:14 PM 10/1/99 -0400, you wrote:
      >Are there any horror stories out there involving navigation by GPS
      >of real navigation? Looking for situations where traditional navigation
      >would probably have saved the day when GPS fouled up.
      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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