Walter Boyd wrote, in part:``The iFly airspaces are defined by a collection of lat/lon waypoints which describe the shape. ....... for our alerting. I consider those definitions more accurate than the Sectionals & Tacs; those charts were originally designed for visual, non precise navigation - so the chart designers would commonly move things around to make room for labels and icons. But they seem to be getting better with each cycle.``
Hi Walter,
I don`t doubt the accuracy of what you report, but FWIW, I`m certain the difference, if any, between the accuracy of the depiction on Vector vs. Sectional view (in the case of the class-D airspace boundary at the MFR airspace where I did the inflight disparity experiments) is clearly _not_ sufficient to explain the approx 0.75 consistant nautical mile disparity that both I and ATC noticed between where iFly was telling me I was relative to their perimeter and where ATC was seeing.
Confirming IMO that is this: I would flip between Sectional view and Vector view the two views agreed as to where the airspace boundary was as precisely as is possible to read an ifly screen. They both show the perimeter at 4.3 nautical miles from the center of of the airport (which happens to be right at 5 statute miles.)
On the other hand, in the case of the disparity at MFR is clear that it can be fully accounted for by iFly`s sectional and vector views (and presumably the FAA) showing the perimeter at 5 statute miles and ATC believing/using 5 nautical miles as their perimeter. All the findings in the experiment are 100% consisent with that.
At the risk of repeating what I imply above: I`d say don`t count on switching to using vector view to get you and ATC to agree when you`ve entered their airspace, especially around class-D where there seems to be some confusion on part of some controllers about whether they should be looking out 4.3 nautical miles (as they do at KLMT per my talks with ATC) or are looking out 5 nautical miles.
A remaining question to me is this: ``How common might it be for a class-D ATC to be using 5 nautical miles as its perimeter radius while the sectional and vector on iFly (and presumably other GPS) is showing 4.3?
Those of you often fly near class D, it would be interesting, at least to me if you`d be willing to do some experiments and report back here: Set iFly to route you to that airport as a waypoint or destination(so iFly`s digital distance estimate to waypoint tells you pretty accuractly how far you are from the airport center) then fly to the perimeter as shown on moving map (sectional and/or sector display) and note what iFly reports as you distance. If ATC isn`t to busy ask them what distance they show you at AND ask if they show you inside our outside their perimeter. If you want to do this without incurring on their airspace (probably prudent) remember you can fly their perimeter 1000 feet (or whatever) above their airspace.
Alex