In the US, the current NAVD88 datum is tied to a defined elevation at one point rather than to any location's exact mean sea level.
Heights for measured points are shown on National Geodetic Survey data sheets,[3] data that was gathered over many decades by precise spirit leveling over thousands of miles.
They may also adopt slightly different but similar definitions for their reference surface.
Since gravity is not constant over large areas the orthometric height of a level surface (equipotential) other than the reference surface is not constant, and orthometric heights need to be corrected for that effect.
[5] Practical applications must use a model rather than measurements to calculate the change in gravitational potential versus depth in the earth, since the geoid is below most of the land surface (e.g., the Helmert orthometric heights of NAVD88).