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

The Flytec 6030/5030/5020 (and comparable Brauniger models) record pressure altitude with respect to standard pressure (1013.25 hPa or 29.92 inHg). This is a requirement for certification and IGC. It also makes sense with respect to air regulations and evaluating altitudes from a scoring standpoint. Since all tracks are recorded with reference to standard pressure then all pilots tracks can be fairly evaluated with respect to each other regardless of the the reference altitude an individual pilot set. If you want Alt1 to show the altitude being recorded in the tracklog then the reference altitude for Alt1 should be set to 1013.25 hPa. This can be done with a press of the function key when setting Alt1 and since the tracklog is recording the altitude regardless of user-setting changing the reference for displayed altitude can even be done in flight :-)

It is my opinion that the proper way to evaluate altitude ceilings is from properly recorded pressure altitude as discussed above. Some have argued that we should use GPS altitude (which these instruments also measure and record) for evaluating altitude limitations in competition. While in certain circumstances GPS altitude can be more accurate than pressure altitude it is not the reference altitude accepted by aviation authorities. A pilot will have been deemed to have violated an airspace ceiling/floor based on pressure altitude regardless of GPS altitude.

The other problem with GPS altitude is that, as you have observed, different GPS models can read markedly different altitude readings when at the same elevation for at least two reasons. First, difference in the quality of signal reception from satellites can cause one unit to have more position error. Secondly, Garmin GPS that do not have a pressure sensor, appear to give GPS altitude, whereas, Garmin GPS that have a pressure sensor initially set the altimeter based on GPS altitude and then rely on the pressure sensor to measure changes in altitude. I suspect that when you were noticing significantly different displayed GPS altitudes with Gerolf one of you had a GPS with a pressure sensor. Garmin GPS that have pressure sensors (e.g., 76S, 76CS, 76CSX, eTrex series, 60 series, Geko 301, etc.) are very commonly used for scoring yet they do not record GPS altitude or pressure altitude! To make matters worse, flight scoring programs seem to treat the altitude values in the tracklog recorded by these devices as GPS altitude. As you have reported, we have seen how this has given an unfair penalty to pilots not using this type of GPS.

Another argument against using GPS altitude is there are only a handful of GPS and or flight instruments that display GPS altitude. In the Garmin product line, all of the commonly used models that do are quite old and will likely be discontinued. It is my belief that the best way to proceed with altitude enforcement is to rely on properly recorded pressure altitude. However, until such time that it becomes a requirement that competitors use flight instruments that properly record pressure altitude altitude, enforcement of altitude limits will be extremely difficult for the scorekeeper or patently unfair to competitors.