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Subject: Re: Electronic vs non-electronic
From: Trayfors, William (btrayfors@XXX.XXX)
Date: Fri Oct 01 1999 - 16:52:00 EDT
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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