This article explains the difference between orthometirc (geoid) height and ellipsoidal height.
The orthometric (geoid) height of a point of the Earth Surface is the distance Ho from the point to the geoid.
The ellipsoidal height of a point of the Earth Surface is the distance He from the point to the ellipsoid.
Earth surface, ellipsoid and geoid
Orthometric height H
o and ellipsoidal height H
e
The geoid height above the ellipsoid (N) is the difference between the ellipsoidal height and orthometric (geoid) height.
N = He − Ho
Where:
N = geoid height above the ellipsoid
He = ellipsoidal height
Ho = orthometric (geoid) height
More information on this topic can be found in the following articles: