russ_watters said:
I would think that gps apps for aviators would make the correction for you.
...and actually, your phone probably has a barometric altimeter in it anyway.
In aviation, we do not use absolute altitude much. We use air pressure, corrected for local barometric readings, while flying below 18000 feet. When flying above 18000 feet, they set the altimeter to 1013 millibars or 29.92 in.Hg. This is referred to as pressure altitude. An altitude reading based upon pressure alone can have errors due to temperature deviations from standard atmosphere.
So when comparing altitude to GPS in flight, there will be errors induced by standard lapse rate deviations. The reason aviators continue to use pressure measurements is because aircraft performance is actually tied quite closely to the concept of pressure altitude. A further correction can be made by introducing temperature corrections as well. This is called density altitude and it is commonly used at high altitude airports. Aircraft landing and take-off distances are directly proportional to it. Climb rate performance is directly affected by it. Engine performance is tied to it. However, in cruising flight, because we can't know the actual temperatures of the air below us, we use pressure measurements and assume a standard lapse rate. Pilots know to reset altimeters frequently when flying from high pressure to low pressure areas, or from warmer to colder air masses. Otherwise they might be closer to terrain than they intend to be.
Thus a GPS reading of absolute altitude is not only inaccurate, but it is also irrelevant in most cases (except possibly for terrain avoidance). And even in the latter case, corrected pressure based altimetry is easier to use (simpler, updates faster, already there, already in use) than a GPS.
(Yes, I've been an instrument rated pilot for many years)