GEG wrote:

Is that alt info built in?

Yes.

If so, is there no correction for barometric pressure changes?

No.

I don't know that it would be severe, but it seems like the
window could be +/- 100 feet from day to day that the controllers
get from the transponder.

True. The equipment at the controllers' end corrects for barometric pressure
fluctuations, so the info the controller sees is pretty accurate.

George Patterson
There's plenty of room for all of God's creatures. Right next to the
mashed potatoes.