Well as ADS-B reports barometric altitude then this is based on the Standard Atmosphere. The Standard Atmosphere is defined in geopotential feet. So that is presumably your answer. Though as barometric is intended to give a relative separation between aircraft in what is only ever an approximation to Standard Atmosphere, and which bears little relation to height above the surface I am not sure where this becomes important.
As a matter of interest...
ADS-B can in principle report Barometric or Geometric altitude in the main position report. However barometric is used pretty much universally for obvious operational reasons.
However: When reporting barometric in the position report, suitably equipped aircraft will report "difference from barometric" in the velocity report. This is a 7 bit field plus sign-bit and is limited to 3125 ft difference between barometric and geometric.
So you actually get both for your money.
Why height information in the velocity report? Well there was a bit of space going spare, while the position report is crammed full!
Last edited by Dont Hang Up; 9th Mar 2016 at 08:41.