which USB GPS device should I buy?

[Mike Oliver]

Quoting Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org>:

> One thing GPSs are NOT good at is altitude.  It was not designed to
> provide that with any kind of accuracy.  If you get +/- 100m you are
> doing well.

That's a little pessimistic in my experience.  The time average of the
altitude reading, over, I don't know, half an hour or so, is usually
good to within 50 ft or so.

> This is why there are a number of hiking GPS units that have altimeters
> built in to get altitude.  It is much more accurate than GPS will ever be.

The best ones have barometric altimeters that automatically recalibrate
themselves based on the GPS altitude reading.  That takes care of the time
averaging automatically, while allowing for you to go up and down.

My Garmin Vista HCx does that, and I'm *mostly* happy with it.  My only
complaint is that, the longer it's been on, the more it trusts the barometric
reading, and therefore the slower it recalibrates itself.  
Qualitatively that's
a good idea; quantitatively it recalibrates too slowly when it's been on for
a while.  Air pressure can change faster than it seems to take into account.

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