Originally Posted by
SNS3Guppy
Meaning what, exactly? If the assertion is that GPS is a better source of altitude information, then I ask again, why is it not used for RVSM?
LH2's comment more fully was - Barometric pressure corrected for the local aircraft effects through the ADC (i.e. the 'consistent' pressure level) is used for RVSM because it is generated internally and has no requirement for reference outside the aircraft. In addition, the outside air pressure is something that can be measured quite accurately with multiply redundant pressure sensors and therefore has a high degree of resilience.
All aircraft at the same point in space will measure the same pressure (subject to minor calibration variances in an RVSM aircraft), hence this is a very robust system for separating aircraft.
Finally, the whole vertical separation infrastructure is based on pressure levels and ubiquitous and multiply redundant GPS is still not with us - so if you were going to define the equipment requirement for RVSM 20 years ago the only choice was barometric systems.
If in 20 years you were going to define a system from scratch to avoid impact with the ground and other aircraft, I would be very surprised if barometric altimetry (subject to human error and very large errors in absolute altitude) would be chosen over EGNOS/WAAS GPS (works without human interaction to an accuracy of a few meters)