This polar motion should not be confused with the changing direction of the Earth's spin axis relative to the stars with different periods, caused mostly by the torques on the Geoid due to the gravitational attraction of the Moon and Sun.
The exact distance varies along meridians because the shape of the Earth is slightly oblate.
Topographic elevation is defined as the vertical distance above the reference geoid, a precise mathematical model of the Earth's sea level as an equipotential gravitational surface.
Undulation of the geoid, the separation between the geoid and the reference ellipsoid of the Earth