iFly GPS Forum

Actually that was my first impulse. I was a bit nervous that an inquiry could be interpreted as me being adversarial, which I was not. Would not want to get on the wrong side of these controllers as I fly you in the near their space. Turns out as you say no reason to avoid contacting.

Call yesterday evening. One controller on duty but not busy.
Happy to chat.

In brief:

Not surprisingly, he said this disparities such as i reported are not uncommon. Many times someone using their GPS reports as 6 miles out when they see them at 7 or whatever. He agreed it's highly unlikely my GPS was off three quarters of a mile, and that he's never figured out the source of the disparity and would be very curious to understand it himself. Best. We could come up with in our blind leading the blind discussion was there could be a disagreement between the sectional and their radar about what constitutes the centerpoint of the airport/airspace.
He said he'd be real interested in having a pilot flying the pilots's GPS indicated line of the airspace boundary some time and he track it on the radar.

So we have an agreement that I'm going to go out there and fly there sometime when they're not too busy and do my best to exactly fly the GPS indicated as airspace boundary ... but for safety do so about 1500 feet above their airspace.
He he went so far as to give the other controllers my N-number and explain our proposed experiment, do there would be no confusion or static when I was doing this.

Should be very interesting. Will report the results here. My best guess is we may find two circles that are slightly offset from each other, perhaps as much as a mile.

I appreciate all the caveats everyone else here posted (along the lines of "never trust the position and altitude anyone,including ATC , reports as precisely accurate, let alone precisely sink with Sarah GPS altimeter readings and cut sufficient slack for potential large disparities")........ seem worth remembering.

Alex

GPS as you say isn't as good at altitude as horizontal..... But when comparing what iFly reports to known surveyed altitudes I am at (on runway, etc) I have never seen it off by more than 50 feet. By contrast even when recently having been reset to nearest officially reported pressure I've seen my altimeter read , IIRR, as much as 150 feet off a known reference altitude .

As for "Since transpoders all report pressure altitude, this error really doesn't matter...we're all working off the same error.":
Unfortunately you can't count on that Walter. Remember, a few of us are flying without transponders. I could be reporting my altitude off my GPS , the tower could be telling you they see me at 4500 however their radar reads altitude (since they get no transponder signal from me), or o could be reading it of my altimeter which, unless by coincidence is set it standard pressure, is as you know giving a different number.

Disappointing, although useful, to hear that we won't know if ADSB traffic altitude is reported as GPS or pressure altitude. Guess the prudent practice will remain ... As one guy here said... If within 500 feet assume traffic is AT my altitude.

Alex