Which instrument directly measures altitude?

The cabin altimeter directly measures altitude by comparing the ambient air pressure with a standard reference pressure. As an aircraft ascends or descends, the change in atmospheric pressure is recorded by the altimeter, allowing it to provide a readout of the aircraft's altitude relative to sea level or a specific reference level. This instrument is critical for pilots to maintain safe altitude levels and adhere to air traffic control instructions.

In contrast, the vertical speed indicator measures the rate of climb or descent rather than altitude itself. The airspeed indicator displays the speed of the aircraft through the air, and a gyrocompass provides directional information based on the Earth's rotational axis, neither of which relates directly to measuring altitude.