I was looking at my altitude graph in SeeYou for my flight yesterday. I have set my Flytec 6030 to display GPS altitude (actual real GPS altitude) by pressing and holding the ALT1 button and then pressing the GPS altitude soft button on the right. I love this function. I didn't make any changes or settings to the pressure altitude.

When my GPS pressure read 7946' (in SeeYou) the "baro" altitude was 7464'. About a 500' difference. My GPS altitude was recorded and displayed in SeeYou at 8967' while the "baro" altitude was recorded and displayed to be 8425'.

When I was down to 704' AGL (I don't know which altitude measurement was used to determine that), my GPS altitude was 1552' and my pressure altitude was 1506'. So it makes a lot of difference if you are up high but less if you are about to land.

Here is what Andrej says about altitudes and how they are calculated in SeeYou from the IGC file:

GPS altitude is as accurate as it is. It is a pretty accurate measurement of altitude above sea level in meters/feet. Its problem is that it is not as responsive as one would like for final glides and similar calculations (admittedly this is less of a problem for paragliders and even hang gliders due to the lower glide ratios). And most of all it is not representative of Flight levels which is what commercial aviation is based upon. The 18,000 ft limit in the US is FL 180, which, of course, depends on pressure systems and temperature.

Pressure altitude covers the latter part of the GPS altitude problem and is very responsive (again admittedly that becomes important on final glides with 25:1 ratio and better). But it is not a very good measure of actual altitude above sea level or the ground because it can drift quite significantly even during a single windy day.

Flight recorders such as Bräuniger Compeo/Competino and Flytec 5030/6030 are supposed to record standard pressure altitude (1013.25mBar) in the IGC files. Given the »Automatically adjust altitude to > QNH« is set, the pressure altitude at landing point is compared with the altitude of the ground at the landing point. This difference is then applied to the whole flight and all pressure altitudes are corrected with this difference.

In addition to the corrected pressure altitude SeeYou also stores the recorded pressure altitude from the IGC file in memory. Depending on whether an airspace limit is AGL or FL it will use the correct altitude to compare whether or not the airspace was infringed or not. It will use recorded pressure altitude to compare to FL limits and the corrected one to compare to AGL types of airspaces.