Altimetry

Altimetry is a techique for measuring the 'height' of something relative to the entire Earth rather than just local features or terrain.

This is usually done by using Satellites that orbit above the Earth based on the overall centre-of-gravity of the Earth. So when the satellites measure the distance from them to the surface below this is relative to the centre of the Earth, not just some local feature.

Definition courtesy of IPCC AR4.