iFly GPS Forum

"We've added a new Altitude AGL instrument. Now there is Altitude MSL and Altitude AGL."

This seems a bit misleading, if not dangerous. Unless you have pitot/static information and a current altimeter setting, the only altitude that can be provided (IIRC) is altitude above the WGS84 ellipsoid. So not "Altitude MSL" at all. In contrast I think the terrain databases have MSL altitude as their reference, right? For terrain warnings, the consequent errors are probably not significant because there is a large safety margin. Providing an AGL "instrument," however, is another matter. Terrain following while scud running or on an approach has little or no safety margin.

Comments? Am I not understanding something? Have you done any calculations to see what the error range in these "instruments" might be?

(For clarity...what I mean is the Terrain database may be "high". The AGL instrument may report false-low values if you are, for example, right next to a montain peak...but it won't report false-low)

I think this is somewhat of a moot point. IFly GPS is not legal for IFR flight and VFR flight rules still pertain. There are acknowledgements of this that we all agree to when we use this app.