Walter Boyd wrote:I am confident this is not a flaw in the iFly, nor in the FAA data we're using. The data we're representing in vector mode, and what we're using for airspace alerting, matches perfectly with FAA Charts (Sectionals, TACs, etc). You can test this yourself: Pull out a ruler and measure a class D on your sectional, then measure it on the iFly....they will be the same. And try this: Fly around the permiter of a class D using pilotage or any other aviation GPS, and compare your position with the iFly: They will match.
Harkening back to the original post, I believe the root issue must be one of two things:
1) The tower in question is using a different radius for their Class D boundary than what the FAA has published.
2) The tower is inaccurate in it's distance measurement.
Hi Walter,
I'm confident iFly knows exactly where it is very accurately in GPS coordinates.
As for "2"....The tower swears by its distance measurement, ATC mentioning to me that they have to calibrate every day.
I don't know what their calibration is. I IMAGINE they have known surveyed ground based metal targets (there is at least one metal tower that happens to be right at the edge of their airspace) and if they aren't showing up at the surveyed distance they would know their radar was off.
As for your suggestion "1":
I don't know ATC at MFR uses as their radius. Asked ATC if the distances they were reporting to me were in statute miles or nautical and they said, as I expected, "statute miles."
Per the post from Jim they and the sectional should be using 5 statute miles, which is 4.35 nm. But when iFly's digital distance-to-airport was showing 4.3 nm, and their radar was reporting ~4 nms, and the iFly moving map was showing me right on their perimeter, ATC was telling me that they saw me as about 3/4th of a mile inside their airspace.
Most confusing to me.
If they ARE using 5 nautical miles as their radius, the results my experiment gets makes sense.... as the sectional is most likely using 5 statute miles as the radius.
Maybe I'll phone them and ask.
I don't know if I can get another GPS for the experiment you propose. But I don't doubt that the position shown on iFly and any other decent GPS will match.
The question of interest to me is would another aviation GPS show my aircraft visually on the moving map as flying outside the visually displayed perimeter while ATC and the GPS's own digital distance-to-airport readout is showing me as inside.... closer than 5 nm.
I'D LOVE IT IF SOME OTHER IFLY OWNER WOULD REPEAT MY EXPERIMENT WITH COOPERATION OF THEIR LOCAL ATC AND REPORT BACK HERE.
(To recap the experiment: By prior arrangement with ATC, I flew as precisely as I could right on the moving map's display of their airspace perimeter with them reporting to me via radio contact at several (about 7) points what distance their radar was showing me at. Then on the phone after the flight we compared notes.)
There is something to me not yet explicable.
To me it's no hazard as I've been duly educated now to not take as gospel the moving maps display as to where ATC is going to see an incursion....But what ever the cause/explaination, it could be a hazard to a pilot unaware.
And it would be at LEAST good to know if this anomaly is common to many airspace boundaries or just an oddity here at MFR.
Alex