A uniform reference became a necessity not only for operational reasons but also essential for aircraft design. The standard atmosphere was derived from the average conditions for all latitudes, seasons and altitudes. The properties of a standard day are related to sea level at latitude 45 degrees with absolutely dry air. To assure altitude separation, all aircraft altimeters are calibrated under ISA conditions. Since the ISA parameters are hypothetical in nature, they constitute an imaginary atmosphere.
Pressure altitude is an imaginary altitude that is used for vertical separation.
Since it is not practical for aircraft that fly long distances at high altitude to constantly change their altimeters to a local setting, it is agreed that above a certain altitude all aircraft use pressure altitude as a uniform altimeter setting. Each country publishes its own transition altitude. A transition altitude is the altitude above sea level where pilots must set their altimeters to standard pressure. When flying with the altimeter set to standard pressure, the altitude is expressed in flight level. Flight level is obtained by removing the altitude's three rightmost digits.