NavList:

I believe that it is possible to separate aircraft accelerations from gravity:

(LINZ = Land Information New Zealand aka Lands And Survey)

"LINZ says that understanding the height of our land is important for understanding how water will flow during a flood or storm. People commonly talk about metres above sea level as a way of describing heights, but sea level can vary at different points around the country. To give scientists and researchers consistency, LINZ has developed a new vertical datum, a reference system based on gravity.

This has been four years in the making and has involved flying the length and breadth of the country with a specially equipped plane for gathering gravity measurements, as gravity can vary from place to place."

For the audo on which this quote is based go to

http://www.radionz.co.nz/national/programmes/ourchangingworld/audio/201807946/exactly-where-is-sea-level-gravity-can-tell-us

I have no idea what technology is used and what it cost.