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Subject: Re: Electronic vs non-electronic
From: John Simmonds (johnsimmo@XXX.XXX)
Date: Mon Oct 04 1999 - 03:05:37 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
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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