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What is the difference between altitude MSL and AGL?

Mean Sea Level (MSL) is the average water level of the ocean, or seas, between high and low tides.  Since this is the same level world-wide, it is referenced as a zero point for height measurements requiring a standard/known zero point.  Height above MSL is used to describe altitude of airplanes, elevations of hills and mountains, and anything else requiring a standard reference point.  If a local zero point or ground level reference is used, heights above this point are called altitude Above Ground Level (AGL).