Geoid Height Calculator
Estimate the geoid undulation (N) at any latitude/longitude (decimal or DMS) from a coarse EGM96 grid, or enter your own N from GEOID18 or EGM2008, then convert between GPS ellipsoid height (h) and orthometric height (H) with h = H + N.
Geoid height (undulation) N is the separation between the WGS 84 ellipsoid and the geoid, so orthometric height H = h − N, where h is the GPS ellipsoid height.
Use the Geoid Height Calculator
Location
Decimal or DMS · −90 to +90 · S is negative
Decimal or DMS · −180 to +180 · W is negative
Built-in model: coarse 5° EGM96 estimate, not survey-grade — use "My own N" with a GEOID18 or EGM2008 value for survey work. Calculation stays in your browser; entered coordinates are masked from session recordings.
Geoid Undulation (N)
EGM96 coarse approximation
Ellipsoid height (h)
Orthometric height (H)
Height Relationship
Conceptual only (not to scale): h = H + N
Formula
h = H + N
- h — ellipsoid height (GPS altitude, WGS-84)
- H — orthometric height (above mean sea level)
- N — geoid undulation (this calculator)
- 1 ft — exactly 0.3048 m
Geoid height at major cities: EGM2008 vs EGM96
N in meters from GeographicLib GeoidEval (EGM2008 and EGM96 grids), compared with this tool's 5° estimate. Orthometric height H = h − N.
| Location | Lat, Lon | EGM2008 N | EGM96 N | This tool (5°) | Error |
|---|---|---|---|---|---|
| New York, USA | 40.7128, -74.006 | -32.72 | -32.76 | -33.66 | -0.94 |
| Los Angeles, USA | 34.0522, -118.2437 | -35.18 | -34.77 | -34.05 | +1.13 |
| Chicago, USA | 41.8781, -87.6298 | -33.93 | -33.94 | -34.06 | -0.13 |
| Denver, USA | 39.7392, -104.9903 | -17.28 | -17.00 | -17.46 | -0.18 |
| Houston, USA | 29.7604, -95.3698 | -28.41 | -28.48 | -28.24 | +0.17 |
| Seattle, USA | 47.6062, -122.3321 | -22.80 | -22.41 | -19.45 | +3.35 |
| Miami, USA | 25.7617, -80.1918 | -27.18 | -27.73 | -28.86 | -1.68 |
| London, UK | 51.5074, -0.1278 | +46.09 | +45.96 | +45.57 | -0.52 |
| Tokyo, Japan | 35.6762, 139.6503 | +36.78 | +36.79 | +34.33 | -2.45 |
| Sydney, Australia | -33.8688, 151.2093 | +22.36 | +22.47 | +21.85 | -0.51 |
| Colombo, Sri Lanka (geoid low) | 6.9271, 79.8612 | -97.20 | -97.65 | -102.07 | -4.87 |
| Port Moresby, PNG (geoid high) | -9.4438, 147.1803 | +74.89 | +74.20 | +74.30 | -0.59 |
| Mount Everest summit | 27.9881, 86.925 | -28.49 | -28.74 | -41.62 | -13.13 |
Reference values: GeographicLib GeoidEval, retrieved 2026-10-09. The 5° grid smooths mountain ranges and steep geoid slopes, which is why Everest and Colombo show the largest errors.
Which geoid model should you use?
| Model | Coverage | Use it when |
|---|---|---|
| GEOID18 (NOAA NGS) | Contiguous US, Puerto Rico, US Virgin Islands | Converting NAD 83(2011) ellipsoid heights to NAVD 88 elevations that must match US benchmarks |
| EGM2008 (NGA) | Global | Accurate WGS 84 heights worldwide; the common choice in GIS and GNSS software |
| EGM96 (NGA) | Global | Legacy datasets, many GPS receivers and aviation systems; this tool's built-in estimate |
Look up an official value with NGS GEOID18 computation or GeoidEval (EGM2008), then paste it into My own N to convert heights with it.
Worked example
A GNSS receiver in New York reports ellipsoid height h = 15.000 m. EGM2008 gives N = −32.722 m, so H = h − N = 15.000 − (−32.722) = 47.722 m above the geoid (156.57 ft). With the built-in 5° estimate (N = −33.66 m) the result would be 48.66 m, about 0.94 m too high.
Quick answer
Geoid height (undulation) N is the separation between the WGS 84 ellipsoid and the geoid, so orthometric height H = h − N, where h is the GPS ellipsoid height. In the contiguous US N is negative (about −17 m in Denver to −35 m in Los Angeles), so mean-sea-level elevation is higher than the GPS ellipsoid height.
Examples
| Location | N (EGM2008) | GPS h = 100 m → H |
|---|---|---|
| New York | −32.72 m | 132.72 m |
| Denver | −17.28 m | 117.28 m |
| Los Angeles | −35.18 m | 135.18 m |
| London | +46.09 m | 53.91 m |
| Colombo (geoid low) | −97.20 m | 197.20 m |
Enter coordinates (decimal or DMS), pick a city, or use your device location, then add a GPS or MSL height to convert.
Summary
A geoid is the equipotential surface of Earth's gravity field that best approximates mean sea level. GPS receivers report ellipsoid height (h) relative to the WGS-84 ellipsoid, but engineering and surveying work requires orthometric height (H) — the height above the geoid (mean sea level). The relationship is h = H + N, where N is the geoid undulation. This tool estimates N for any latitude and longitude from a coarse EGM96 grid, or takes an N you looked up in an official model, then converts between h and H.
How it works
- Enter latitude and longitude in decimal degrees (40.7128, −74.006) or degrees-minutes-seconds (40°42'46"N, 74°00'22"W), pick a city, or use your device location.
- Built-in source: N is bilinearly interpolated from official NGA EGM96 values sampled every 5°. Error is often under 2 m in lowlands but can exceed 10 m in high mountains.
- Own N source: paste the geoid height from NOAA GEOID18, GeographicLib EGM2008, or your survey software to convert exactly with that model.
- Ellipsoid height entered (GPS): H = h − N. Orthometric height entered (MSL): h = H + N.
- Results are shown in meters and international feet (1 ft = 0.3048 m) with a diagram of the height relationship.
Use cases
- Convert raw GPS altitude readings to mean-sea-level elevation for mapping.
- Apply a GEOID18 or EGM2008 geoid height you looked up to GNSS or drone heights.
- Check geoid undulation before importing drone survey data into engineering software.
- Understand why GPS elevation differs from a topo map contour value.
- Provide a quick N estimate for field surveys before accessing a full geoid grid file.
- Teach geodesy students the ellipsoid–geoid–topography height relationship.