SEA LEVEL

Measuring GCP elevation from the center of the Earth is impractical so instead we measure elevation from the more useful reference, sea level.

As sea level is constantly changing, this extremely complicated! However, geodetic scientist have created a model that can define sea level to within 1 meter anywhere on earth. The accomplish this by building a geopotential mode of the Earth consisting of spherical harmonic coefficients. This model is called a geoid and it is tells us where the theoretic sea level is everywhere on earth.

Check out the video below for an excellent overview of how sea level is measured.