I know that the Garmin 305 uses barometric pressure in some way for the altitude. That works quite well under normal circumstances, but in good ole flat as a pancake So Fla, it leads to some unusual anomolies.
See this ride report.
http://ridewithgps.com/trips/18674 **
** I later when back and had the site fetch USGS data and it corrected the elevations.
The actual elevation profile for the entire ride should be similar to the first 17 miles of the ride... dead flat. Then a pretty sever front moved thru (and yes we got soaked.)
Note that the elevations went wacky! At one point it changed 100 ft while I was drinking coffee at Starbucks! I know I got a little caffeine high, but....
Is there any site that downloads correct the barometric pressure data based on NOAA records for the time, date and GPS location? I know from my days Air Force days that pilots would call in for an altimeter check and get the current barometric pressure to correct their altimeters.