Indicated altitude is the altitude that is corrected for local atmospheric conditions. It is shown and directly read off the altimeter.
Indicated altitude is used to maintain terrain/obstacle clearance and to maintain vertical separation as planes pass over each other.
The definition of indicated altitude is easy enough to remember, but should you also keep in mind a few other things, including how to best set the altimeter, and what can affect the indicated altitude reading.
4 Things You Should Know About Indicated Altitude
1. It’s best to set the altimeter to the airport elevation if you are on the ground. At this point indicated altitude is the same as the true altitude (set to QNH which is sea level pressure). Indicated altitude should always be used for traffic separation.
2. Keep a lookout for when indicated altitude is different from true altitude. This most commonly occurs when flying in cold temperatures. The lower the temperature, the more inaccurate the reading will be.
3. Remember that the actual altitude reading will be lower than what the altimeter states in colder than normal conditions. In warmer conditions, the actual altitude will be higher than the indicated altitude stated on the altimeter.
4. Beware of mountain flying and the Venturi effect. This is when an isolated low-pressure area forms, and causes a significant difference between indicated and actual altitude. The indicated altitude could be as much as 1,000 feet higher in fact.