I thought the GPSs used triangulation for altitude. I didn't know they had pressure transducers. Are you sure? The altitude seems very stable, i.e. it doesn't seem to go up and down with barometric pressure by a thousand or more feet. If they use a pressure sensor then why do they show 2D NAV sometimes? This doesn't mean Latitude and altitude but no longitude! I'm sure the MLR uses triangulation. Fairly accurately now they switched the skew off.
GPS uses ranging or distances (intersecting spheres), not angles (triangulation), to determine where we are (try http://www.garmin.com/manuals/gps4beg.pdf for a reasonable overview). It calculates the distance from each satellite to the receiver using an idea of the correct time and information in the broadcast signal. In simple terms, four satellites provide a unique solution for the position of the receiver, giving your 3-D position in space plus time as the unknowns in the equation. Yes, we don't need barometric altimeters, since we generally have more than 4 satellites: GPS can provide it all, IMHO. But some GPS receivers have barometric altimeters built-in - because I guess people want them.
[Some receivers will allow you to do 2-D positions (the Garmin 12 for example), I guess that time is being given priority over the Z here.]
Barometric altitude: well, I've been flying with a well-known brand vario this summer next to my Garmin eTrex Vista. After a few flights I found the max altitude always higher on the GPS than the (vario) altimeter, even with both re-set at TO. The difference was about 30m over 1000m of climb. My first reaction was that the GPS was doing something strange, but then I checked that my GPS would auto-calibrate, etc. and tried again: same problem. More worryingly, when I landed (700m below TO) I found I had the reverse problem: (vario) altimeter reading too high.
My conclusions:
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The eTrex Vista track log height is GPS calibrated, can be left alone to do its stuff, and is accurate enough.
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Even though the Vista has a barometric altimeter built in, the altitude in the track log seems to be calibrated to the GPS "height" if you ask for it. It looks like GPS height (maybe Garmin can confirm).
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The eTrex is probably more reliable than a barometric altimeter; my vario altimeter calibration curve gives 3% error in climb/descent heights
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I could probably get rid of my "bulky" vario/altimeter and have a audible only in my helmet and my eTrex instead (smaller, less wind resistance)
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At least two current instruments seem to be heading this way? Brauniger's Galileo and Digifly's Graviter?
With respect to data logging, there seems to me no reason in this day and age to use barometric pressure altimeters for anything except ensuring you stay out of air lanes and controlled airspace (the flight level of which is usually barometric standardised pressure). If we rely on the GPS to give position for a data log, we should make it 3-D. Wasn't the use of barometers in the past to show that you remained airborne for a certain period of time? So let's not get hung up trying to cling on to that technology. With the advent shortly of WAAS and EGNOS (sorry Australia) there will be good reliability for GPS 3-D up-time, too.
Why does the Vista have a barometric pressure altimeter? Best ask Garmin, but if I was a sailor or a mountaineer, then I'd be interested in pressure change - tells you lots about weather trends. That's inferior to the GPS though if you have actually changed your altitude and you need to record it; you could expect something like +/-10m absolute error 95% of the time (see http://celia.mehaffey.com/dale/dgps.htm), whereas the barometer goes up and down with weather changes.
Some other useful links:
Height from GPS: http://www.edu-observatory.org/gps/height.html
Accuracy of height from GPS: http://users.erols.com/dlwilson/gpsvert.htm
1:52:45 PM 
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