Under what condition is indicated altitude the same as true altitude?

• A. When at sea level under standard conditions.
• B. At all altitudes in standard atmosphere.
• C. When temperature is nonstandard.
• D. In high humidity.

Answer: (A)

Indicated altitude comes from the pressure the atmosphere provides and how the altimeter is set, while true altitude is the actual height above mean sea level. They line up only when the atmosphere is the same as the International Standard Atmosphere and you’re at the reference level where the standard pressure applies. The simplest case is sea level under standard conditions: the ambient pressure equals the standard sea‑level pressure, and with the altimeter set to standard, it reads zero altitude, which matches the true height above sea level. In any other situation, differences arise because actual pressure and temperature deviate from standard, shifting where pressure surfaces lie. Temperature deviations affect true altitude relative to indicated (warmer air raises the pressure surfaces, cooler air lowers them), while humidity has negligible effect on the reading.