ATC and iFly disparity in location? - iFly General Discussions

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HomeHomeDiscussionsDiscussionsiFly General Di...iFly General Di...ATC and iFly disparity in location?ATC and iFly disparity in location?
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6/11/2014 1:36 PM
 
Cobra wrote:

.....If you're saying the circle drawn on the map representing the class D region is ~4.3 nm in radius, then that's a mapping issue independent of aircraft position measurement.

Is that what you're saying?


Yes, as far as it is possible for me to tell from fairly careful experiments, both from what ATC is reporting when I fly the perimeter and what iFly is reporting as distance to the center of the airport, the radius of the airspace shown on both the Sectional View and the Vector View appears to be at about 4.3 nm .
If so (and I believe it is) you can call it a mapping issue..... but it could be an issue with how iFly shows the map or plane on the display.

Bottom line at the moment is if a pilot trusts the moving map display to be showing him/her to be reasonably safely, say, almost a mile outside the airspace he/she may well be inside and not know it.

I do believe it would be good to drill down deeper into this.
Whatever the source of the misleading info on the moving map display, it's not good to be looking there and seeing you're not inside airspace when you are inside.


Alex
P.S., Correction by Alex added per Jim's later post:
The normal radius of class D airspace is 5 statute miles, NOT five nautical miles.
That's 4.35 statute miles, which is right where iFly and its moving map is showing the perimeter.
So all the discrepancy I have observed COULD be perfectly explained if the local ATC is treating their airspace as having a 5 NAUTICAL mile radius. That would be surprising to me, but not impossible.
 
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6/11/2014 1:45 PM
 

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6/11/2014 4:46 PM
 
Jim Fitzgerald wrote:

In my opinion, not that it counts for anything, that would be a serious flaw. Have you tried using other class b, c, or d boudaries to see if you get a similiar error. If you don't, it could simply be bad data for that particular airport. If the error repeats for other class b, c, and d airspace, it sounds more like a program error to me.

Do you have another aviation moving map application you can use as a comparison? I would think they all get their data from the same source, but I don't know that for sure.

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.


Walter Boyd
President, Adventure Pilot
 
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6/11/2014 7:15 PM
 
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




 
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6/12/2014 9:51 AM
 
ComputerDoc wrote:
...(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. ...

Bingo! First, compare the depiction of that tower between ifly and your paper sectional, especially as it relates to the class D circle. Are they the same? If so, fly over or near that tower to verify that ifly shows you in the correct place. I'll bet it does! At least for that one point, that will fully verify the accuracy of ifly.

I spent time yesterday closely comparing my local airspace between the paper sectional and both the sectional and vector views on ifly. I found no issues. I have located some landmarks I intended to compare from the air on today's flight, but unfortunately weather kept me in the pattern. I will still do that, but I don't expect to find any issues.

ComputerDoc wrote:

...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. ...


That's what I asked a few days ago! Somebody's confusion between statute and nautical miles is still the only explanation that makes sense to me. The numbers match perfectly.

Vaughn Simon Nexus 7 with Dual XGPS170
 
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