On Thu, 11 Jul 2002, Richard Lynch wrote:
> NOTE:  End-user GPS units are "off" by like 30 meters (?) on purpose so Joe
> Blow can't tell the "enemy" how to find the White House and blow it up. 
> Sigh.  You're never gonna beat that error margin without a *LOT* of repeated
> readings over a long period of time and statistical analysis...

This is completely off-topic, but the Selective Availability system that
you describe has been discontinued by the Department of Defense a few
years ago - after it began to be undermined by the proliferation of
differential GPS beacons (these are transcievers at stationary points that
listen to the GPS signal, calculate the difference between the delivered
signal and their known location, and broadcast a corrective signal). Even 
the Coast Guard was running them.

So for the past few years consumer GPS is very accurate. This helps
explain the recent rapid spread of consumer GPS devices.

DoD says they reserve the right to turn it on if they want to, but I doubt 
they would - especially now that the EU is planning its own competing 
system.

> But keeping all that junk in the database as 2-D mapping would only be
> needed if you're going to be providing driving directions and keeping all
> the highways and all that crap that MapQuest keeps (I assume).  You almost
> for sure don't need all that.

It seems to me like they still need street addresses, unless they have a 
database of the shapes of every private property parcel in the country.

miguel


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