iFly GPS Forum

It is not immaterial at all.  I fly by my plane's altimeter, not by the Indicated Altitude instrument that iFly is showing on my tablet.  If they differ by 250' then that is more than is allowed even for VFR flight.  (E.g. If ATC asks me to hold an altitude of 5500' and they're actually seeing 5750' then that's a problem as far as they're concerned.  That being said, I've never had them come back to me -- so, I'm inclined to think that the iFly Indicated Altitude is not what ATC is seeing.)  Also, as Cobra pointed out, the correction factor that iFly is applying to the pressure altitude is their choice and it's unlikely to be the same as the one that ATC is using.  But next time I'm up I'll talk to a couple of airports and ATC and ask what they see.  If what they're seeing is that far off my plane's altimeter (which generally agrees with GPS altitude), then I need to get my altitude encoder adjusted or replaced.

Also, part of the reason I posted here is to see if other people are seeing a similar difference.  If they are, then this might be a bug in how iFly calculates Indicated Altitude.

Sorry, this cannot always be true, even if it's what someone at AP told you.  My iFly reports an Indicated Altitude number, but it has no possible method of obtaining that data from my transponder.  I am 100% certain it gets this data from the barometric pressure sensor built into the AHRS board on my Stratux.

I do think that iFly can sometimes get the transponder altitude from ADSB devices that are designed to "sniff" Mode C transponder broadcasts and incorporate the encoded altitude data into their own datastream feeding iFly, but that is not the case in my aircraft.