Interesting thread.
On the iFly (and other apps I'm sure), airspace boundaries come from a detailed collection of lat/lon points provided by the FAA. It wouldn't suprise me (for Class D at least) if the controllers just drew a circle on their radar display with a sharpie, or something equally approximistical.
Regarding altitude...I didn't know radar provided altitude information to the controllers. I wonder if he was just looking out his window to get that number? To the other posters points...there are a few reasons GPS altitude will be different than pressure altitude. These are the two biggies:
- As you all know, we are constantly calibrating our altimeters to accomodate changing pressure. But this calibration is based on a measurement taken at ground level. So unless temps and pressure are perfectly standard, the higher you go, the more "error" you have in actual altitude. 50' per 1000' AGL is not uncommon. Since transpoders all report pressure altitude, this error really doesn't matter...we're all working off the same error.
- GPSes are very good at triangulating horizontally. The satellites you're tracking typically have a large variety of different horizontal distances from your current location, so it's pretty easy to measure and conclude where you are horizontally. But the relative vertical distance between you and the satellites is tiny in comparison. So unless you have a couple of satellites directly overhead, there is very little information to calculate your position vertically (altitude).
I typically assume my GPS altitude will be different than my pressure altitude by 10% AGL. This variation also factors into our traffic alerting system. ADSB traffic will be sometimes be reported as pressure altitude, and sometimes GPS altitude. And we won't know which.
Cheers,
Walter