Originally Posted by baozebub
Uncorrected GPS gives false altitude.
It can at times do that. The Garmin's use the 3 strongest satelitte for your lat/long and then uses the next strongest one to figure out your altitude. While it can track up to 12 satelittes, it only uses 4 max at any second. Most of the time on my rides, the altimeter is accurate within 20 feet (I've checked vs established USGS benchmarks). However, if your reception is poor due to trees, buildings, the most inaccurate part of the GPS will be the altitude. WAAS helps greatly but the WAAS signal is easily lost and WAAS is only most effective on the east and west coast of the US.
Overall, I found the altimeter to be very useful on the GPS. But if want the best accuracy a barometric barometer calibrated with GPS corrections is what is needed.