Re: GPS and American River

["Duncan Fletcher" <dfletche@gte.net>]

Not sure when the ride occurred. This is important. The feds turned off
Selective Availability on May 2. Selective Availabilty was the deliberately
introduced inaccuracy for those without the military software to correct for
it. That can introduce even higher error factors in the odometer reading.
The slower the speed, the greater the error in the odometer reading. In fact
it can accumulate miles while standing still. With SA turned off, that error
should be much smaller. It is also possible that with temporarily poor
satelite coverage it went into 2D mode which is much less accurate. That can
result from being in a canyon, from tree cover, or from body shielding
depending on the GPS is carried.

Duncan Fletcher
dfletche@gte.net


----- Original Message -----
From: "Dorothy Joiner" <djoiner@countyofplumas.com>
To: <Ridecamp@endurance.net>
Sent: Friday, May 12, 2000 1:13 PM
Subject: RC: GPS and American River


We rode the 30 mile ride at American River.  Just for fun, we decided to
take our GPS and set the odometer mode so that we would know exactly how far
we'd traveled.  The GPS and the mileage map were almost exactly the
same...until the last 10 miles.  At that point, according to the GPS, we had
gone about 2 miles further than where they said that check was (Rattlesnake
Bar).  We chocked that up to going off the trail, down to the water, etc.

However, when we got to the end of the ride, our GPS was reading 39
miles--not 30.  The folks we talked to there swore up and down that our GPS
was wrong.  So we checked it in the odometer mode travelling in the car.
Its right on.

Any ideas what would cause a GPS to ADD miles?  I know they can sometimes
cut them off if you don't have good sattleite reception--but add??

Dorothy & Elly
Butterfly Valley