What is geoid undulation?

Geoid undulation (N) is the vertical separation between the WGS-84 reference ellipsoid and the gravity-based geoid. The relationship is h = H + N, where h is ellipsoid height and H is orthometric height. N may be positive or negative.

How accurate is this calculator?

This calculator bilinearly interpolates EGM96 values sampled at 5° × 5° from NGA's official grid. It is intended for education and coarse preliminary estimates. Error varies by location, so it is not survey-grade.

Can this replace a regional geoid model?

No. EGM96 is a legacy global model. Regional models such as NOAA GEOID18 or AUSGeoid2020 may connect to official local vertical datums and are not interchangeable with this estimate.

How are feet converted?

The calculator uses the exact international-foot definition: 1 foot = 0.3048 meters.

Why does my GPS show a different elevation than a topo map?

GPS reports ellipsoid height above WGS-84. Topo maps show orthometric height above the geoid (sea level). The difference is the geoid undulation N, which can be tens of meters. Applying the N correction from this tool should bring GPS altitude close to the map elevation.

Can I use decimal degrees with south or west coordinates?

Yes. Use negative values for south latitudes (e.g. −33.87 for Sydney) and for west longitudes (e.g. −74.01 for New York). The calculator accepts any value in the valid range.

Geoid Height Calculator

Estimate the geoid undulation (N) at any latitude/longitude using a simplified EGM96 model, then convert between ellipsoid height (GPS) and orthometric height (elevation above sea level).

Location

Latitude (decimal degrees)

−90 (South Pole) to +90 (North Pole)

Longitude (decimal degrees)

−180 (west) to +180 (east)

Known height (optional)

This height is:

Ellipsoid height (GPS / h) Orthometric height (MSL / H)

Legacy model: coarse 5° EGM96 estimate, not survey-grade. 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

Summary

Estimate the geoid undulation (N) at any latitude/longitude using a simplified EGM96 model, then convert between ellipsoid height (GPS) and orthometric height (elevation above sea level).

How it works

Enter a latitude (−90° to 90°) and longitude (−180° to 180°) in decimal degrees.
The calculator looks up the geoid undulation N from a built-in EGM96 approximation grid.
Bilinear interpolation refines N between grid nodes for smoother results.
If you enter an ellipsoid height (from GPS), the tool computes orthometric height H = h − N.
If you enter an orthometric height, it computes the ellipsoid height h = H + N.
Results are shown in meters and international feet with a visual summary of the height relationship.

Use cases

Convert raw GPS altitude readings to mean-sea-level elevation for mapping.
Verify orthometric heights in GIS projects that mix GPS and leveling data.
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.

Frequently Asked Questions

Last updated: 2026-09-23 · Reviewed by Nham Vu