I have an old Garmin III and am upgrading to a Magellan Meridian Platinum (on the way via UPS and delayed do to snow back east) because I wanted the electronic compass and electronic altimeter. Both of these features (in the Magellan) can be switched off at any time and the unit will switch to triangulation for both.
Anyway, I did a lot of research before deciding to go with the Meridian and finally decided to go with it do to the very fact you have been talking about in this thread. It seems that the flaw in the Garmin altimeter is just what you have mentioned, it uses the barometric pressure and certain allogrhythm (sp) for their altimeter to work and this can change drastically with the weather changes (see great reviews on GPS systems at this site, have been using it for years and they are spot on.. http://gpsinformation.net/
All the major GPS web sellers recommend this site for product reviews and they are non bias. See the Garmin GPS Map 76S review for what they say (they say it a lot better than I can).
Magellan uses a totally different (and much more accurate) function for their electronic compass and altimeter and the accuracy (according to the reviews) is dead on. And, both functions can also be switched off to go back to satelite triangulation. Garmin has not changed even with their newest models and according to the reviews is a big flaw in this function of their units with electronic compass and electronic altimeters. The one nice thing about this function in the Garmin units is that at a certain speed (not very fast) their units automatically switch from electronic to the GPS triangulation function.
I have been using my Garmin III hiking for years and have found it to be very accurate but very outdated (especially in the battery eating catagory).. thus the reason it upgrade. I looked at many units (Garmin and Magellan combined) and decided to go with the Meridian unit, even though it is an older model) because according to the reviews it still offers more standard features than Garmin does in their more expensive and newer models.
Here is the excerpt from the review directly from Garmin in answer to their question on this problem (note their statement several times "After the calibration" and "remains stationary after the calibration" (in other words, you have to recalibrate it every time you want an accurate reading)..
"The normalized pressure is only valid immediately after a calibration. In order for us to accurately compute it after a calibration, we would need to have an independent source of elevation. What the unit does subsequent to the calibration is to continue to assume that the unit is remaining stationary at the last calibration elevation, and that any ambient pressure variation that is measured is due to atmospheric pressure change, which causes normalized pressure to change as well.
You should see the altimeter functioning very accurately after you calibrate it, and that is our priority in this product. Normalized pressure was provided so it could be used to allow a user with accurate knowledge of his elevation to compute a normalized or sea level pressure. The normalized pressure that is displayed is NOT the
pressure that is used in the standard atmosphere model to compute elevation. There is no loss of accuracy in the altimeter portion of the unit just because the normalized pressure we display is incorrect. Remember, normalized pressure is based on the last calibration elevation.
With the recent release of National Geographic Topo software for the Mac I will also be plottiing and charting my hikes on my computer to check for accuracy. I have done a couple small hikes since moving to Prescott 3 weeks ago and charted them and it works great.