Height datum relations
The vertical datum of a height reference system is usually determined by the mean sea level, which is estimated by one or more tide gauges of an adjacent sea. The tide gauge stations of the national height systems in Europe are located at various oceans and inland seas: Baltic Sea, North Sea, Mediterranean Sea, Black Sea, Atlantic Ocean. The differences between these sea levels can come up to several decimeters. They are caused by the various separations between the ocean surface and the geoid.
Landlocked countries in particular determined the relation of their height systems to a sea level by leveling observations to neighbouring countries. These measurements were in some cases already done in the 19th century and were accordingly inaccurate.
The below picture shows the reference tide gauges for European national height reference systems and the offsets to EVRF2019 in cm.
In Europe different kinds of heights are used (normal heights, orthometric heights, normal-orthometric heights). They differ in the method, how the leveled height differences are corrected because of the gravity of the Earth. In some European height systems, heights without any gravity correction are used.
The following picture shows the kind of heights, which are in use in European countries.
More detailed information about European coordinate reference systems and their transformation parameters to the pan-European Vertical Reference System EVRS are available at Information System for European Coordinate Reference Systems CRS-EU.