>> All the information re GPS has been helpful, thanks everyone. Another question though....how close to your target do they really put you?? Cheryl >> The GPS specification for SPS (Standard Positioning Service) is to be within 100 meters 95% of the time and within 300 meters 99.99% of the time. You can expect many GPS units to just about cut these figures in half, however, especially if you average your position for about 15 minutes. See http://users.erols.com/dlwilson/gpsavg.htm for more information than you want to know about the subject. In an experiment I conducted a few weekends ago, I left my Garmin 12XL on while skiing cross-country trails on the edge of the Boundary Waters. When I got home, I uploaded the resulting tracks to the TOPO! mapping program. The greatest divergence between tracks (coming and going on the same trail, where they should have coincided) was about 100 meters, and that might have been because the batteries were running low (they were exhausted by the time I got back to the cabin). On another loop, which I skied on two different days, the two sets of tracks were practically superimposed (within about 5 meters of each other). One thing I noticed from this experiment is that the GPS positions seem most accurate where I was moving, probably because the Garmins try to predict your course for a short distance ahead, to allow for momentary gaps in satellite reception. In places where I stopped for several minutes, such as at shelters, there is a small cluster of dots caused by a deliberate degradation of the signal called Selective Availability (SA). I have a JPG file that shows this GPS track on a topo map, and if I had a place to post it on the Web, I would. Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Thu Mar 02 2000 - 11:12:29 PST
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