Bruce Kavanagh <<email>> writes:

Turning off Auto-Calibrate will not help. It might even make matters worse depending upon how it works - which is not explained in the manual. The problem is that the Garmin's with a pressure sensor do not record GPS altitude but instead record Pressure altitude. The two are fundamentally different things. In a standard atmosphere (which we never actually fly in) they are the same, but the hotter it gets the more they diverge. That article I sent a few months ago explains why. Depending upon what altitude the launch was at and what the lapse rate was, a temperature of 43C on the ground means that 11ft of GPS alt = approx 10ft of pressure altitude. 1ft is not always 1ft when you are talking about pressure altitude!

They are two fundamentally different quantities and so it is simply not acceptable to use GPS altitude for some pilots and pressure altitude for others (which is what happens if using Garmin's with a pressure sensor, and GPSs without a pressure sensor). It's like using the straight line distance for some pilots and the total tracklog snail trail distance for others when scoring their distance.

The difference that you saw will have mostly been due to the fact that they are measuring different parameters rather than any error from either instrument.

Steve Kroop <<email>> of Flytec USA writes:

I believe turning off Auto-Calibrate turns off the GPS altitude completely. The problem with GPS altitude is that it is slow to update. This is why they put the pressure sensor in. The only time the Garmin uses the GPS altitude is to set the pressure altitude on start up or after a significant period of nearly no altitude change (so while your taking a lunch break on a mountain hike the altimeter will auto correct the pressure altitude). I believe turning of auto correction disables this feature.

Looking at the Gecko 301 this afternoon, it was clear that you turned Auto-Calibrate off if you wanted to set the altitude at a known altitude on launch. The Gecko 301 with Auto-calibrate on sets the Baro altitude to the GPS altitude that it measures when you are sitting around at launch. After that the Baro altitude is recorded.

So it would appear that "all" modern GPSes do not record GPS altitude. GPS altitude is not something that comes out of a GPS. If you are turning in your GPS for a height determination along with your task, you are getting the benefit of a lower altitude reading because the baro altitude is lower (see above).

Pilots who turn in actual GPS altitude (from Flytec 5030 and 6030's, for example) are disadvantaged by maybe up to 500', because they turn in an actual GPS altitude.