Even with the correct altimeter setting your barometric altitude can differ from your true altitude
by as much as 10 percent.
Airport, terrain and obstacle elevations on aeronautical charts are true altitudes
Absolute altitude is the height above terrain computed by subtracting terrain elevation from true altitude
In these cases, unless the altimeter setting is adjusted, the height of the pressure level corresponding to the current altimeter setting, and therefore, true altitude
, will decrease--we'll be lower than we realize.
Remember that heavy smokers can increase their true altitude
by 5000 to 8000 feet when considering oxygen use and TUC.
Your indicated altitude is corrected for non-standard pressure, but would also need to be adjusted for non-standard temperature as well as pressure differences between your position and the reporting weather station in order to obtain your true altitude
. Oh, and the altimeter is electric, so it's not reliable anyway.
Altitude is true altitude
(your actual height above sea level) as determined by your GPS position and is quite accurate even when compared to a properly set altimeter.
In practice, nonstandard temperatures affect all aircraft equally, so all aircraft at an indicated altitude will still be on the same horizontal plane, even though the true altitude
of that plane shifts with temperature.
The fact that the area he is flying in is not designated as mountainous does not change the fact that it is mountainous, and altimeter errors due to venturi effects, etc., can make the indicated altitude higher than true altitude
. Some local knowledge and experience may allow one to make proper judgments about these effects in the specific location, but in general, one should be cautious and conservative.
The FB Winds are provided to pilots at true altitudes
every 3000 feet through 12,000 feet and pressure altitudes every 6000 feet at or above 18,000 feet.