NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Electronic vs non-electronic
From: Craig Scott
Date: 1999 Oct 03, 19:24 EDT
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? 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
