It strikes me that with GPS altitude and weather soundings and
measurements, with just the GPS WAAS altitude it should
be possible to calculate pressure altitude.

If so, and this were reliable, there would be no need for
a power-hungry mode C transponder, one could use a GPS based
transponder and the ATC computer could simply spit out
altitude. This isn't so farfetched since that same computer
already compensates for pressure differences.

On the other hand, with a reliable automated way to get
these soundings to the transponder, the transponder
could make these calculations BEFORE spitting out the
calculated pressure altitude.

I wonder how close the altitudes calculated this way would
match the altimeter? I think this would be a matter of how
good the soundings are at helping this process.