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Subject: Re: Electronic vs non-electronic
From: Craig (craig@XXX.XXX)
Date: Sun Oct 03 1999 - 19:24:48 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?
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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