I’ve seen a lot of confusion from people when determining what the difference is between AGL & MSL. The confusion does not only exist within the IFC, but there are outside cases of this confusion. For people struggling to determine the difference between the 2, I have created a tutorial to eliminate the confusion.
AGL = Above Ground Level
MSL = Mean Sea Level
The purpose of AGL:
AGL is the most commonly used. It references your altitude you are at from the ground below you. So say I am at FL350 above the DFW metropolitan area. My altitude is set at FL350, however the ground above me ranges from 400 to 1300 feet. So that means as I am cruising over the DFW metropolitan area, my altitude will constantly be changing, no matter what I set my altitude at. That is used a lot when flying. The only time you won’t usually be using it is when you are flying in mountainous areas, such as Salt Lake City and Denver.
The purpose of MSL:
MSL, as stated above, is used in mountainous areas. It is also used at higher altitudes, and over seas and oceans. When you are flying over the alps, or Himalayas, oceans, or areas like that, it is important to use MSL. MSL references your FL above the ground below you. When an aircraft has reached an altitude where AGL is harder to use, the pilot will determine his altitude off of MSL.
I hope my tutorial today has helped you understand the difference between the 2 altitude references and when you should use each one. Thanks for taking the time to read this tutorial, and I hope you got something out of this today! Here is a little additional resource to use:
Thanks @Chris_S for the pic.