While flying from KSNA to KVCB VFR at night, Norcal Approach asked for my current altitude. I was somewhere between 12.0 and 12.5 depending on which reading I was going to use. ATC had me confirm. My GPS altitude was 12.5, my transponder was showing 12.3, and my altimeter indicated 12.0. I am assuming, since altimeter settings are based on the barometric pressure on the ground, that the thinner air above 12,000 feet would make my altimeter the least reliable of the three and the GPS the most reliable.
When ATC asks, should I give them the altimeter indicated altitude? Also, is the transponder altitude that I can see derived from secondary radar and isn't that what ATC is already seeing?