A transition altitude shall normally be specified for each aerodrome by the state in which the aerodrome is located. Where two or more closely spaced aerodromes are located so that coordinated procedures are required, a common transition altitude shall be established. This common transition altitude shall be the highest that would be required if the aerodromes were considered separately.
The height above the aerodrome of the transition altitude shall be as low as possible but normally not less than 900m (3000ft). Transition altitudes shall be published in aeronautical information publications and shown on the appropriate charts.
System of Flight Levels
Semicircular Cruising Level System (ICAO)
Cruising Levels Applicable in RVSM Airspace