I've published two articles on the use of height restrictions in competition over the last few days, here and here. And there is an ongoing and much involved discussion going on here.

3D GPS's have become a requirement for some high level competitions lately. We made them a semi-requirement for the 2007 Worlds so that we could use them to see if pilots were cloud flying. They would be used to see the altitude differences between pilots who notified us that a certain pilot had gone into a cloud, and the pilot who may have gone into a cloud. We never had to use them (mostly because it was mostly blue during the World).

For our purposes GPS or pressure altitude would have worked as long as all the pilots were using the same altitude type. We would have most likely used GPS altitude which would have been fine, because we weren't dealing with any air space restrictions.

Other meets have had height restrictions because of airspace considerations. Air space altitudes are based on pressure altitudes, not on GPS altitudes, but because of the limitations of GPS based scoring systems, GPS altitude has been used, without much fore thought, to determine if pilots have broken air space. Pilots who rely on flight instruments that display barometric altitude, have been staying out of airspace but have penalized for going above altitude limits that have been measured by GPS altitude, when they should have been measured by pressure altitude.

Pilot's track logs, IGC file, that originate in sophisticated flight instruments such as the Compeo, Compeo+, 5020, 5030 or 6030 include both pressure altitude and GPS altitude, but most scoring systems are only looking at GPS altitude to determine if the pilot broke the air space restrictions. I use my SeeYou based scoring system, and it displays both GPS and pressure altitude, so I can determine if a pilot actually did break airspace.

But other scoring systems, FS for example, don't display pressure altitude. In addition, unlike SeeYou, they don't have a map database, that allows for the pressure altitude to be adjusted for the actual launch or landing elevation, if the pilot didn't make the correct determination on launch. This would have to be done manually by the scorekeeper for pressure altitude.

Some pilots are using GPSes that have pressure measuring sensors, some GPSes don't have them, so only GPS altitude is available from them. Apparently some GPSes are reporting pressure altitude as GPS altitude.

We have been using GPS based scoring systems successfully for quite a few years now, but we took that success and pushed it over the edge when we required 3D GPSes and assumed that the same scoring systems could be used to tell use about altitude as well as position.

I suggest that before we continue penalizing pilots for breaking airspace, we thoroughly review this problem to see if we have not jumped the gun a bit. We did not use height restrictions in the 2008 pre-Worlds basically because the threat of them was enough to keep pilots out of the clouds.

A technical discussion of these issues can be found here and the following post