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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
John
> 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
>      To: NAVIGATION-L@XXX.XXX
>      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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