MSL vs Ellipsoidal Height: The Confusing World of Vertical Datums
Ask a GPS receiver for your elevation and it gives you a number. Ask a traditional level survey for elevation at the same spot and you often get a different number, sometimes by several metres. Neither is wrong. They are measuring height relative to two completely different reference surfaces, and confusing the two causes real errors.
Mean Sea Level (MSL): The Traditional Reference
MSL height, also called orthometric height, is measured relative to the geoid, a complex model of earth's actual gravitational equipotential surface, closely approximating average sea level if oceans covered the entire earth. This is the "elevation" engineers have traditionally used for centuries.
Ellipsoidal Height: What GPS Naturally Measures
Just as this background reflects real time, GPS naturally measures height against a mathematical model, not the sea you actually see.
GPS satellites calculate position relative to a simplified mathematical ellipsoid model of the earth, not the complex geoid. This ellipsoidal height can differ from MSL height by anywhere from a few metres to over a hundred metres depending on location, due to local gravity variations.
Practical Impact on Survey Work
| Scenario | Correct Approach |
|---|---|
| DGPS survey feeding into engineering design | Apply correct regional geoid model for MSL conversion |
| Comparing GPS data with old traditional survey | Confirm both are referenced to the same vertical datum |
| Cross-border or multi-region projects | Verify consistent geoid model across the full extent |
Every DGPS deliverable we produce applies the correct geoid model conversion, ensuring elevation data is genuinely comparable with existing engineering drawings and legacy survey data. See our DGPS survey services.
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