Andy...
You are quite right... BUT, the ATC equipment will convert the Mode C to an ALTITUDE (if you are at or below the Transition Alt)
Which means that a/c one at 4800ft may be indicating 4600ft on radar (within the allowable tollerances) and a/c two is indicating 5000ft, but actually at 4800 as well... clang.
Bear in mind that in the USA the transition alt is somewhat higher than over most of the UK..... which is another argu... no, no, no I meant Thread.
rgds BEX
