Which altitude is defined as the actual altitude above mean sea level (MSL)?

The correct choice is true altitude, which is defined as the actual altitude of an aircraft above mean sea level (MSL). This measurement is particularly important for ensuring safety during flight, as it allows pilots to determine their actual vertical position relative to the Earth's surface.

True altitude is typically obtained through precise navigation systems and altimeters calibrated to reference mean sea level. It is essential for navigation, obstacle clearance, and compliance with air traffic control instructions, as pilots need to maintain a certain altitude relative to MSL.

While pressure altitude is based on standard atmospheric pressure and can vary with weather conditions, indicated altitude refers to the altitude read directly from the altimeter without adjustments for temperature or pressure changes. Absolute altitude measures the aircraft's height above the ground or terrain beneath it, which can differ significantly from mean sea level based on local geography. True altitude specifically represents the vertical distance above the standard reference point of mean sea level, making it the most reliable measurement for aviation standards.