Two surfaces, two zeroes
A GNSS receiver calculates its position relative to the WGS 84 ellipsoid, a regular geometric form that approximates the Earth. The altitude on a map usually starts from a surface tied to gravity and mean sea level.
EGM2008 describes that second surface globally. In Romania, the model’s geoid lies above the ellipsoid, so height above mean sea level is lower than raw ellipsoid height.
In the equation, h is height above the ellipsoid, N is the local separation between the two surfaces and H is modelled height above mean sea level.
The correction changes across the map
In the east, near the Danube Delta, the separation falls to about 30.2 m. In the south-west it reaches about 45.3 m. Read to one decimal place, the ends of the range are 15.1 m apart.
45.3 − 30.2 = 15.1 m
The variation follows the Earth’s gravity field, which reflects how mass is distributed within and across the planet. The correction therefore depends on location, rather than on the altitude of the mountain, building or receiver.
Know which value you have
A phone or receiver may already show a corrected altitude. Check the device label and reference system before using the equation: subtracting N applies to a raw ellipsoid height compatible with WGS 84.
EGM2008 provides a global approximation of mean sea level. Surveying, cadastral and construction work uses official benchmarks and vertical reference systems, with accuracy requirements beyond the scope of this explanation.