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.
Two different heights
GNSS natively reports height above the ellipsoid, a mathematical surface. Construction uses orthometric height, measured from the geoid, which approximates mean sea level. The two can differ by tens of metres.
The geoid model bridges them
Orthometric height equals ellipsoidal height minus the geoid separation for that location. Without the right geoid model, the conversion is a guess.
Why it matters on site
Mixing the two produces levels that look plausible and drain the wrong way. Never subtract one type of height from the other.
Check on a known benchmark
Measure a benchmark with a published level before starting. That one reading tells you whether your height chain is set up correctly.
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