Ellipsoidal and orthometric heights
Why an RTK elevation can differ from height above sea level by tens of metres, and how to apply a geoid model to get orthometric heights.
RTK Premium corrections give your receiver ellipsoidal heights by default. If you need height above mean sea level, known as orthometric height, apply a geoid model in your field software, rover, or GIS.
Why your elevation can look wrong
An ellipsoidal height is measured from a mathematical model of the Earth's shape, not from sea level. The difference between the two, called the geoid separation, can be tens of metres depending on where you are. A horizontal position that is correct to the centimetre can still show an elevation that looks far off if you expected an orthometric value.
The offset is close to constant across a site, so every point is shifted by about the same amount. That pattern points to the height reference, not to the correction stream.
How to get orthometric heights
Confirm which geoid model your project requires. The national mapping agency for your country publishes the official model.
Select that geoid model in your field software, rover, or GIS.
Measure a known control point and compare its height with the published value before relying on elevations.
Heights and the antenna
A wrong antenna height produces a similar symptom: the same vertical offset on every point. Check the antenna height entry before changing the geoid model. See Inconsistent RTK accuracy on site.
FAQ
Why is my RTK elevation tens of metres off? The receiver is most likely reporting ellipsoidal height and you expected height above sea level. Apply the geoid model for your region.
Does RTK Premium deliver orthometric heights? The corrections give ellipsoidal heights. Your software converts them to orthometric heights once you select a geoid model.
My plan position is right but heights are off by a few centimetres. Where do I look? Check the antenna height entry first, then confirm that the geoid model and its version match what your project requires.
For what affects vertical accuracy in the field, see Accuracy expectations.
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