Orthometric elevation, ellipsoidal height and geoid: why GNSS Z is not always site level
A GNSS receiver gives a Z value, but that does not automatically make it the construction elevation. Vertical references need the same care as horizontal CRS.
Field takeaways
- GNSS height is commonly handled as ellipsoidal height.
- Construction levels usually need orthometric elevation tied to a vertical datum.
- TUCBS describes the relation as h = H + N.
- TUDKA99 is the Turkish vertical reference frame tied to Antalya mean sea level.
- Record vertical datum and control checks with Z values.
Right in plan, wrong in height
You measured with RTK and the E and N sat beautifully on the control point. Good news — but it does not mean the Z can be used with the same confidence. The GNSS vertical component is weaker than the horizontal, and beyond that you need to know which kind of height you are holding. On site, "level" means height above the sea for most people; a GNSS receiver natively reports height above the ellipsoid, a mathematical surface. The geoid bridges the two, and the difference can run to tens of metres.
The three quantities
The TUCBS document defines ellipsoidal height as the distance from the point to the ellipsoid along its normal, and orthometric height as the height above the geoid along the plumb line. The practical relation reads h = H + N: ellipsoidal height equals orthometric height plus the geoid separation. More important than memorising the formula is knowing which of the three your device and software are showing — and never subtracting one type of height from the other.
TUDKA99 and the Antalya connection
On the vertical reference side in Türkiye, TUDKA99 is the key frame: the TUCBS summary ties it to mean sea level at the Antalya tide gauge, with an orthometric height system. The message for the field: when site levels are discussed, what matters is not just the number the receiver shows but which vertical reference it has been converted into.
Checking a Z value in the field
- Ask which height the project wants: ellipsoidal or orthometric?
- Note the geoid model and conversion method used.
- Check the GNSS Z against a benchmark with a published level.
- Plan levelling or total station support for critical level work.
- State the Z reference explicitly in every export.
Keeping sources honest
When producing profiles, areas, volumes or point lists in MapLab Survey, keep the Z column's origin explicit: RTK measurement, total station data, a levelled and checked value, or a scouting-grade phone height. If sources mix in one file, separate them by layer or note. In field reality, the best data is the data you can still defend later — and mixed heights produce levels that look plausible and drain the wrong way.
Try these steps on your own phone.
MapLab Survey handles RTK/GNSS point capture, drawing, area and volume calculations and coordinate conversion, then exports to DXF, KML/KMZ, GeoJSON, Shapefile, GeoPackage, NCZ or CSV. Capture, calculation and export work offline. Live NTRIP corrections and online basemaps need a connection.
Frequently asked questions
Is GNSS Z always site elevation?
No. Check the vertical reference and height type.
What does h = H + N mean?
It links ellipsoidal height, orthometric height and geoid height.
What is needed for critical levels?
Known elevation control and often leveling or total station support.
Technical references
The field guidance in this article is aligned with the technical and official references below.
Related: Creating an elevation profile · RTK GNSS field accuracy · TUCBS CRS · MapLab Survey