Educational post regarding altitudes for radar vectors

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.


Great post but I thought that pattern instructions were always given in AGL?

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Any altitude giving by ATC is Above main sea level.

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Does anyone know what hight every color represent on the ATC maps?
that could give approach/centre a beter idea to what level they could assign a plane to when mountains are crossing the flight path?

Not sure about the ATC map but when I’m using synvision in Fore Flight, Obstacles or Terrain more than 1000’ below you are colored green; within 1000’ below your altitude are Yellow; and within 100’ below to above your altitude are Red.

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Thanks Aernout, and again great post!

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should try out fore flight my self soon, perhaps I get the answer. Did some ATC in Denver region and noticed a complete different color scheme as flying my home amsterdam region (where -5 is the standard ;-) and no hills) and you can let descent to any FL :smiley: safely

great post Aernout…

When using the ATC map click and hold on a plane to bring up the pink director line. Then drag it over the terrain that you want to look at a height for. It will pop up in that box that shows degrees and milage.

Very useful post Aernout. One minor detail, AMSL is Above Mean Sea Level, the middle of the range of measured sea levels.

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I use the pink line most of the time to vector as it is much faster. It is especially helpful when working appr. At KPSP working around the mountains.