What does true altitude measure?

True altitude measures the height of an aircraft above mean sea level (MSL), which is a standardized reference point used in aviation. This measurement provides an accurate and consistent way to determine an aircraft's altitude, allowing for safe navigation and communication between pilots and air traffic controllers.

When flying, pilots use altimeters set to the local barometric pressure at sea level to ascertain their altitude; this is known as pressure altitude. True altitude takes this a step further by indicating how high the aircraft is above the average level of the ocean, thus providing a more precise and universally understandable altitude figure for safety and operational procedures.

The other options do not directly define true altitude. For instance, measuring height relative to the ground refers to absolute altitude, while altitude corrected for temperature and pressure pertains to density altitude. Navigation-related altitudes may involve different considerations and have specific definitions that do not correlate to true altitude.