Altitude 1 : Height above a reference point, usually expressed in feet. Reference points are typically sea level, the ground, or airfield elevation in which case MSL, AGL or AFE further describes the altitude, respectively
United States Department of Transportation, Federal Aviation Administration, Final Environmental Impact Statement: New York/New Jersey/Philadelphia Metropolitan Area Airspace Redesign, Chapter Seven: List of Acronyms, Abbreviations and Glossary of Terms In This EIS, Volume One: Documentation, Washington, DC, USA 2007. http://www.dirtyairport.com/dotnetnuke/LinkClick.a spx?fileticket=bnCberFBJnk%3D&tabid=56&mid=372, (Avn Sfty - Amer) {1015}