Geoid Height Calculator (h = H + N)
Enter any two values to calculate the third. All values are in meters.
What Is Geoid Height?
Geoid height (also called geoid undulation, usually written as N) is the separation between the reference ellipsoid and the geoid at a given location. In practical terms, it helps convert GPS-derived heights into elevations that are meaningful for engineering, mapping, and surveying.
The standard height relationship is:
h = H + N
- h = ellipsoidal height (from GNSS/GPS)
- H = orthometric height (height above mean sea level)
- N = geoid height (geoid undulation)
How to Use This Calculator
This calculator is designed for quick field and office checks.
- Enter any two of the three values: h, H, and N.
- Click Calculate to solve for the missing variable.
- If all three are entered, the calculator performs a consistency check and reports any mismatch.
- Use the decimal place box to control output precision.
Example Calculation
Given
- Ellipsoidal height, h = 152.438 m
- Orthometric height, H = 121.007 m
Find
Geoid height, N
Compute
N = h − H = 152.438 − 121.007 = 31.431 m
This means the geoid is 31.431 meters above the ellipsoid reference at that location (depending on sign conventions and local model definitions).
Why Geoid Height Matters
GPS receivers naturally output ellipsoidal heights, but most real-world projects require elevations referenced to mean sea level. Without geoid correction, elevation values can be off by tens of meters depending on region.
- Surveying and boundary work
- Civil engineering and construction staking
- Flood modeling and drainage design
- GIS and terrain analysis
- Infrastructure monitoring
Sign Conventions and Unit Notes
Units
Keep all inputs in the same unit system. This calculator assumes meters, but the formula works with feet as long as every value uses feet.
Signs
Geoid height may be positive or negative depending on where the geoid lies relative to the reference ellipsoid. Always confirm conventions used by your geoid model and software.
Accuracy Considerations
This page provides the arithmetic relationship only. True geoid determination requires a geoid model such as EGM96, EGM2008, GEOID18, or local national models. For high-accuracy work, use official model grids and coordinate transformations provided by your mapping authority.
- Model choice affects final elevation accuracy
- Datum consistency is critical (horizontal and vertical)
- Interpolation methods can add small errors
- Field QC should include benchmarks where possible
Common Mistakes to Avoid
- Mixing meters and feet in the same equation
- Using a different vertical datum than the project requires
- Confusing orthometric and ellipsoidal heights
- Ignoring sign on geoid undulation values
Quick FAQ
Is geoid height the same as elevation?
No. Geoid height is the separation between surfaces; elevation is typically orthometric height above mean sea level.
Can I get N directly from GNSS?
Some GNSS software applies geoid models internally, but raw GNSS heights are usually ellipsoidal and require model-based correction.
Can this calculator replace a geoid model?
No. It performs the formula once values are known. It does not estimate N from latitude/longitude by itself.