# 100 metres can mean 54.7

In south-western Romania, a raw GNSS height of 100.0 m above the WGS 84 ellipsoid corresponds to a modelled height of 54.7 m above the EGM2008 geoid. Both values describe the same point.

## 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.

`H ≈ h − N`

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.

## Method and sources

The values come from the EGM2008 gravity model published by the National Geospatial-Intelligence Agency. At each chosen position, the value is interpolated between model points spaced 2.5 arc minutes apart. Romania’s outline is the 2024 edition from Eurostat GISCO.

The national extremes were sought only within the country outline. Values are rounded to 0.1 m, and the result is calculated from the rounded terms so the subtraction can be checked directly.

- [NGA: EGM2008 and the global 2.5-minute grid](https://earth-info.nga.mil/index.php?action=wgs84&dir=wgs84)
- [NOAA: the definition of the geoid](https://www.ngs.noaa.gov/GEOID/geoid_def.html)
- [EPSG: the EGM2008 vertical reference system](https://epsg.org/crs_3855/EGM2008-height.html)
- [Eurostat GISCO: country boundaries](https://gisco-services.ec.europa.eu/distribution/v2/countries/)

The model describes a global gravity-based surface and does not replace Romania’s legal vertical datum. Last checked: 20 August 2026.