Originally Posted by
treadigraph
Ah, OK, I don't usually note whether its ADSB or MLAT...
Saw a relatively new French registered helicopter flying over here several times under low clag last year and reporting an altitude of 1000' - the land rises to 574' ASL locally and I saw it come right over the top of me on one occasion as I crossed that summit - it was very low, 300 - 400' sounds about right. Now my understanding is that F/W aircraft should be no less than 1000' over a built up area and helicopters 900' - can I presume that figure should be AGL and ADSB would be reporting ASL?
It's more complicated than that.
ADS-B reports altitude the same way as Flight Levels are derived - that's to say, based on a nominal QNH of 1013.2 mb (or hPa). So to work out actual height AMSL you need to adjust for the local QNH, and then if you want height AGL you need to factor in the terrain.