What does indicated altitude measure?

Indicated altitude is the reading displayed on the altimeter when it is set to the local altimeter setting, which means it reflects the uncorrected altitude with respect to an atmospheric pressure reference. This measurement is derived directly from the altimeter, which senses the ambient air pressure and translates it into an altitude reading based on the International Standard Atmosphere (ISA).

Essentially, indicated altitude does not take into account any adjustments for pressure differences or temperature deviations that might affect true altitude measurements. Therefore, it represents the raw altitude reading based solely on the altimeter's current setting, providing pilots with an immediate reference for altitude without the intricacies of corrections for weather variations or other factors that could affect the actual altitude above mean sea level or above ground level.