I can hit buffs all day long at anything under 4k yards above me. Over 3.5k yards you can still hit them, but you do not see the "ping". At 4k you can't hit them anymore at all. So, you're only safe at 12k+ "above ground level". The above ground level thing is the important part, not above sea level. Being 12k on your altimeter when attacking a 2k field is only 10k, so you're easy meat.