What is the primary reference used to determine the orthometric height above the geoid?

The primary reference used to determine orthometric height above the geoid is mean sea level. Orthometric height is the distance measured along a plumb line from the geoid to a point on the Earth's surface. The geoid itself is an equipotential surface of the Earth's gravitational field that corresponds closely to mean sea level.

In this context, mean sea level serves as a baseline reference for establishing vertical datums used in surveying. When calculating orthometric heights, the differences between ellipsoidal elevations and the geoid need to be considered. The ellipsoidal elevation is the height above a mathematical ellipsoid, while orthometric height takes into account the local variations in gravity and the actual shape of the Earth.

Vertical datums are established based on mean sea level and can vary by location due to geological and oceanographic factors, but they are used to provide a consistent reference for measurements. Local leveled height is also dependent on reference datums and local conditions.

Understanding orthometric height in relation to the geoid reinforces the importance of using mean sea level as a key reference in surveying practices.