AGL or AMSL- Which to Refer?

Which altitude shall be considered when maintaining the appropriate one according to distance. e.g if we are to say that 3000 ft is the ideal altitude at 10 NM, is it AGL or AMSL?

MSL + Airport Elevation.


Correct me if I’m wrong but this if how I believe it is:
MSL = Mean Sea Level, ie, at the beach midway between high tide and low tide
A(M)SL = Above Sea Level
AGL = Above Ground Level

When approaching an airport you typically want to join the ILS/GPS path into the runway at approx 3000 feet above ground level. So, prior to landing check the airport’s altitude, and make sure you come in at approx 3000 feet (AGL) above airport altitude.

So airport at 425 feet, you approach at 3500 feet AGL.

Hope this helps. If I have it wrong, please correct me and I’ll update this Post.


In your example you want to join the glide slope with 3000 ft above ground level of the airport, i.e. AAL (above airfield/aerodrome level), which may be different to AGL of your position. If the airport is at 450ft (ASL), you should approach at 3500ft ASL (not AGL). If there is no elevation difference between airfield and your ground position, this would still be 3000ft AGL, but you need MSL+airport elevation+3000. For communication between ATC und pilots in your example: 3500. Please correct me, if I’m wrong.

I wish there were altimeters

@azeeuwnl @Cameron_Brown @Laurens @hjo : Thanks for the inputs. I got it, what it means is I need to add the airport altitude to my AMSL to be approaching at the right altitude. By the way, @hjo why have you kept a buffer of 50 ft when you add 450 ft to 3000 ft AMSL? [quote=“hjo, post:4, topic:17505”]
If the airport is at 450ft (ASL), you should approach at 3500ft ASL (not AGL).

Thanks all again!

Good question. Mathematically you’re right - 3450ft ASL would be ok. But my highest resolution of altitude is 500ft, so I just rounded up (same as Anton did in his posting)