Final answer:
Cruising altitude level rules are measured in terms of Mean Sea Level (MSL), which refers to the average height of the ocean's surface. It provides a consistent and universal standard for measuring altitude in aviation.
Step-by-step explanation:
The cruising altitude level rules are measured in terms of Mean Sea Level (MSL).
MSL refers to the average height of the ocean's surface, which acts as the reference point for altitudes in aviation. It provides a consistent and universal standard for measuring altitude, regardless of the location.
For example, if a plane is flying at a cruising altitude of 35,000 feet MSL, it means the aircraft is 35,000 feet above the average level of the world's oceans.