Mark James Boyd wrote:

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.


When restricted to the conditions of usual soaring, no weather sounding
is necessary, we need only a reference point with known pressure and true
altitude. As we soar in constantly rising or sinking air and out of
clouds, i.e. no saturation, we can easily predict the result of the sounding
maybe with a constant offset that the reference point would drop : the
vertical temperature gradient in this conditions is the dry adiabatic lapse
(1°C per 100m) and everything can be computed from this.