In the realm of GNSS positioning and mapping, two reference surfaces coexist: the ellipsoid and the geoid.
At first glance, the distinction seems technical, but for drone operators who produce precise deliverables (DTMs, orthophotos, 3D models), not understanding this difference can lead to errors of several dozen meters in altitude.
Here is why this topic deserves your full attention and how France precisely sets its "zero level" of altitude.
The ellipsoid: a mathematical and smooth Earth
The ellipsoid of revolution is a simple geometric shape: a sphere flattened at the poles. This surface is used to model the Earth uniformly and to calculate GNSS coordinates (latitude, longitude, ellipsoidal height).
The most common is the reference ellipsoid of the WGS84 system, used by GPS systems.
It is defined by:
- An equatorial radius: 6,378,137 meters,
- Flattening: ~1/298,257.
When a drone gives you a GNSS height, it refers to its vertical position relative to that ellipsoid.
The GNSS available in the world and their reference ellipsoids:
| GNSS | Country / Region | Reference ellipsoid |
|---|---|---|
| GPS | 🇺🇸 United States | WGS 84 (World Geodetic System 1984) |
| Galileo | 🇪🇺 European Union | GRS 80 (almost similar to WGS84) |
| GLONASS | 🇷🇺 Russia | PZ-90 (Parametry Zemli 1990) |
| BeiDou | 🇨🇳 China | CGCS2000 (China Geodetic Coordinate System 2000) |
| QZSS | 🇯🇵 Japan (regional) | JGD2000 (Japanese Geodetic Datum 2000 – basé sur GRS80) |
| IRNSS (NavIC) | 🇮🇳 India(regional) | Everest 1830 modifié / GRS80 (in recent usages) |
The geoid: a physical Earth, influenced by gravity
The geoid, on the other hand, is a physical surface: it corresponds to the equipotential surface of the Earth's gravitational field, that is to say, the average surface of the oceans at rest, extended under the continents.
This surface undulates according to the internal masses of the Earth.
It is irregular, but it represents the reference level for "real" altitudes, called orthometric altitudes.
Do you want to see a representation of the geoid "EGM 2008," a global geoid published by NASA and NGA? It's by ici.
The relationship between geoid and ellipsoid
Here is the formula that connects these three concepts:
h = H + N
(ellipsoidal height = orthometric altitude + geoid undulation)
- h : height measured by GNSS (WGS84 ellipsoid reference)
- H : reel height
- N : distance between the geoid and the ellipsoid (called undulation)
So, to convert a GNSS height (provided by a drone or GPS) into true altitude, you need to subtract the geoid undulation.
In France, zero altitude = sea level in Marseille
In France, the reference altitude level is based on a very concrete historical measurement: the tide gauge of Marseille.
Between 1885 and 1897, the average height of the sea was measured there. It is this value that defines the zero point of the NGF system (General Leveling of France), still used today.
From this point, a precision leveling network was deployed across the territory to measure orthometric elevations (those of topographic maps or cadastres).
The NGF-IGN69 system (in mainland France) and NGF-IGN78 system (in Corsica) are the two main national altimetric reference systems.
French geoid model: RAF20
To simplify conversions, the IGN offers geoid models adapted to France, the most recent of which is RAF20. This model allows for:
- Know the N undulation at every point of the territory,
- Easily convert GNSS elevation data to NGF,
- Ensure consistency between your drone data and urban planning documents, road plans, GIS, or public specifications.
The RAF20 model is available on the IGN website and can be integrated into QGIS or photogrammetry software.
The impact on a topographic map or a DEM
An uncorrected GPS altitude can distort an entire digital terrain model (DTM) or a topographic map. Even a vertical offset of a few meters between the ellipsoid and the geoid can cause:
- incorrect ground levels,
- poorly calculated slopes,
- inaccurate excavation/backfill volumes,
- and inconsistencies with the other georeferenced data.
In a professional context, this can lead to design errors, cost overruns on site, and even a loss of credibility with the client or the design office.
Integrating the correct altitude from the start is therefore essential to ensure the geometric accuracy and operational usefulness of the deliverable.
Best practices for your deliverables
- Always check the expected vertical reference (often NGF).
- Apply the RAF20 geoid model in your processing or GIS software.
- Keep track of your conversions in your deliverables (metadata).
Useful references
Next Episode :
Geoid vs Ellipsoid (Part II): How our pilots prepare their missions with ground tracking.
Ellipsoid vs. Geoid: Why Drone Pilots Absolutely Need to Know the Difference?