iFly GPS Forum

iFly actually uses an algorithm to select and periodically update a local barometric pressure from METARs it gets via the ADSB-in feed to drive the "Indicated altitude" reading.  So, if iFly is getting the Mode C transponder-broadcast pressure altitude from an ADSB-in receiver that "sniffs" such data, and if iFly happens to select the same barometric adjustment that ATC is currently using, then iFly will be displaying the same Indicated Altitude that ATC is currently reading.  But only if those two things are true.

Cobra's first explanation is correct.  The Ind.Alt instrument is calculated from Pressure Altitude data sent from an ADSB device, adjusted using nearest METAR to get Indicated Altitude.  ADSB receivers can get the pressure from several sources - Stratux uses an internal sensor, some devices can be wired to the altitude encoder, others passively listen to the transponder signal.  If everything is working properly, it should be pretty close to what your altimeter steam guage is showing.  But there is plenty of room for something to not be working perfectly here:

1. The nearest METAR might not be the best one, if it's on the other side of a front for example
    
2. For a device like a Stratux, using a pressure sensor - I'm not sure how accurate that is - nor do I know how much in-cabin pressure differs. Would be an interesting study.
    
3. The Echo and SkyGuard are examples of devices that passivly listens to the transponder.  One thing not many realize is the transponder only transmits pressure if it's "pinged" by radar.  So if you are in the middle of nowhere, it could be some time between radar pings, hince your altitude appears to not change.  And then it might suddely change a lot.  I've heard some devices simulate a radar sweep that forces the transponder to update more often - but I'm not an authority on this.  
    
4. Some of the Levil devices have very accurate pressure as they are wired into the pitot static system (or like the BOM they have a built in accurate external sensor) - this would be your best bet for accurate Ind Altitude.

Hope this clears things up.  If I missed a question in this thread, let me know.
Walter

Thanks for the info Walter.  But the Ind.Alt instrument number is extremely off in my plane from what they're seeing "on the ground".  Consistently 200-500' higher.  I know  that that's way more than can be accounted for by iFly using a barometric pressure that's a few 100th's (or even a 10th) different from the one that a different nearby tower or ATC is using.  I just removed it from my instrument layout as it was a distraction.

Here's what I found in my long flight outbound on Monday after calling the towers at a Class C, two Class D's, and talking to Seattle Center ATC: if I set my Kollsman window to what the tower or ATC was reporting then my plane's built-in altimeter was spot on with what they were seeing on their displays, but interestingly enough, the GPS altitude as reported by iFly (and presumably obtained from my SkyRadar DX ADS-B In device) was always about 100' higher.  I have to presume that this was due to the round-to-the-nearest-100' that goes on with those displays and barometric conditions at that time.  But it does show the limitations of this old school method everyone(?) still uses to determine altitude.  (I'm still believing that the GPS altitude is WAAS-accurate -- within 3 meters of my actual position 95% of the time.)

[Edit: also, now looking at my MyFlightBook telemetry data which is using the built-in GPS in my phone, it agrees with the iFly GPS instrument that on every leg I was actually higher than what my altimeter, and ATC, were telling me I was flying.]

So, three things: it turns out that my blind altitude encoder must be reasonably accurate; I now know for sure that I should fly by my Kollsman-adjusted altimeter, at least when under ATC guidance; and the adjusted altitude everyone is (should be, probably are) using is only within the ballpark of being the "real" altitude.  Even though I best be using everyone else' altitude frame set (Kollsman-adjusted barometric altimeter) when enroute, when I'm close to landing I'm still going to adjust the altimeter to match the GPS.