AGL: Above Ground Level
AMSL: Above Main Sea Level
For radar vectoring its very important to understand the difference between these 2 definitions as this can be the difference between a CFIT (Controlled Flight Into Terrain) or not.
Whenever you give an altitude while under radar vectoring this altitude is based on AMSL. This won’t be any issue of the airport is at sea level and there is no surrounding terrain like EHAM/AMS.
The problem starts when you are vectoring in an area where there is significant terrain. While your vectored altitude is AMSL, lets say “descend and maintain 5000 feet” and the terrain is 4500 feet high you will end up in a difference of 500 feet AGL between the airplane and the terrain. Keep this in mind as at all stages you never want an airplane to be closer then 2000 AGL except when the airplane is established on the localiser and following the glide slope ofcourse.
Let me give you another example based on KASE, Aspen – Pitkin County Airport. This airport has an elevation of 7820 feet AMSL. So when you are in your airplane you will read you are at 7820 feet Altitude but at the same time you are 0 feet AGL. If you have traffic inbound in KASE and you tell them “descend and maintain 6000 feet” this airplane will be flying straight into the ground (CFIT) because it descends on AMSL altitude.
I hope this is clearing up some problems for you guys I have been reading about on different Facebook pages and the IF community website. Otherwise please do not hesitate to ask or PM.