otnmbrd wrote in message
ink.net...
Jeff Morris wrote:
Eisboch wrote:


I have been told that factory GPS and most high-end retrofit systems
for
cars also use a speed sensor input from the car transmission that is
separate from the GPS signal. It is used primarily to keep position
calculations going when the satellite signal is lost (tunnel) but it
also
helps improve the accuracy of the position readings.


That is the claim for my wife's Toyota nav.

I don't think ours uses a "lock to road" feature - watching the marker
as we drive we were frequently 10 to 20 feet on either side of the road.
Also, it seems to take us "off road" 50 feet going down the driveway.



This is a similar problem on marine chart plotters when doing close-in
work.
Aside from the typical "lag" time, you will frequently find yourself not
exactly where you want to be and/or are. G Gonna be awhile before GPS
takes over completely, but as processors get faster......

otn

Yeah. It's a son-of-a-gun when you think you know where you are in an
unmarked channel and suddenly, as the GPS gets a download update, you find
yourself having moved 30 feet sideways.

Eisboch