Transponders give Flight Level (assuming they are working properly), so
will give the altitude ATC are intereted in. That's how they know what
FL you are at when not talking to them.

At 12:52 26 September 2008, John Smith wrote:
JJ Sinclair wrote:

On the last home-built I regestered, the federallies required a
mechanical airspeed, altimeter and compass........nothing else. But I
have observed that the panel I am now flying has altitude displayed
all over the place, in two GPS's, in the SN-10 and in the
transponder.
The large altimeter takes up a lot of space and provides the least
important info..........................we can all estimate our
altitude for landing, can't we? The rest of the time its just nice to
know info.

Except when you're flying in the among controlled airspace, where
neither GPS altitude (geometric) nor transponder altitude (QNE) do you
any good. Loggers usually can give you QNH altitude, provided you set
them correctly and your battery isn't flat.