Compute geoid undulation using heights, units, and checks. Switch modes for h, H, or N. Export clean reports for field notes and audits today.
Example values are illustrative and may not match your local geoid model.
| Station | h (m) | H (m) | Computed N = h − H (m) |
|---|---|---|---|
| A-101 | 112.3450 | 98.1200 | 14.2250 |
| B-205 | 56.7800 | 62.1400 | -5.3600 |
| C-330 | 245.9000 | 231.4800 | 14.4200 |
Geoid height (also called geoid undulation) links two common height systems: ellipsoidal height h and orthometric height H.
Sign conventions vary across datasets; ensure your h, H, and N definitions match your geoid model.
Geoid height, often called undulation, is the separation between the reference ellipsoid and the geoid. Surveyors treat the geoid as an equipotential surface that approximates mean sea level, while GNSS receivers report ellipsoidal height. Because the two surfaces differ by tens of meters in many regions, converting between them is essential for consistent elevation work.
The calculator uses the standard geodetic identity N = h − H. Here, h is ellipsoidal height, H is orthometric height, and N is geoid height. With any two values, the third is determined. This identity is widely used in engineering surveys, mapping control, and GNSS post-processing pipelines.
Global models show N ranges roughly from about −100 m to +100 m, depending on location and model epoch. Positive N means the geoid lies above the ellipsoid, while negative N means it lies below. Local ranges are usually narrower, but steep gradients can occur near strong gravity anomalies.
Field logs mix meters and feet, so the tool converts both ways and also reports meters for auditing. Decimal places affect readability, not physics. For construction staking, 2 to 3 decimals in meters may be adequate. For high-order control, store full precision and round only in deliverables.
Some projects use local vertical datums, legacy benchmarks, or hybrid transformations. The optional offset lets you model a known shift consistently across repeated computations. Document the offset description in exported files so downstream users understand the applied convention and avoid double-corrections.
Ellipsoidal heights come from GNSS observations, often after antenna and tide corrections. Orthometric heights come from leveling networks or published benchmark sheets. Geoid heights come from national geoid grids or global gravity models. Always note the model name, grid resolution, and epoch used for N.
Large residuals between computed and expected heights usually indicate a sign convention mismatch, wrong units, or mixed datums. Recompute a known control point, verify antenna reference offsets, and confirm whether orthometric heights are referenced to the same vertical datum as the geoid model. Repeat stations help detect outliers quickly.
Capture h from GNSS, obtain H from project control, and compute N for consistency checks. Alternatively, combine h with a trusted N grid to generate H for GIS and design surfaces. Export CSV for spreadsheets and PDF for site records. Consistent documentation is the fastest way to prevent elevation errors.Keep a calibration log, including dates, instruments, and checkpoints, so future revisions remain traceable across teams and contractors.
Ellipsoidal height h is referenced to a mathematical ellipsoid used by GNSS. Orthometric height H is referenced to the geoid, approximating mean sea level. They differ by the geoid height N.
Compute N when you have both h and H for a control point and want a local check. For production work, use an official geoid grid to obtain N consistently over an area.
Negative N occurs where the geoid lies below the ellipsoid for your chosen model and location. It is not an error by itself. Verify unit choice and ensure your sign convention matches the data source.
No. Rounding only changes how values display in reports. Keep full precision in raw records, then round outputs to the tolerance required by your project specifications and mapping products.
It represents a known adjustment, such as a local datum shift, benchmark bias, or agreed project correction. Always document the reason and value so others do not apply the same correction twice.
Yes. Choose feet to enter and view results in feet, while the tool also reports meters for auditing. This helps compare against published geoid grids that are commonly distributed in metric units.
GNSS typically provides ellipsoidal height h after antenna corrections and processing. To obtain orthometric height H, combine h with a trusted geoid height N from an official grid or compute N from control points.
Accurate heights improve maps, engineering, and safe navigation everywhere.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.