Geoid height (EGM96)

Model: EGM96 geoid on a 15′ grid (GeographicLib packaging), cubic or bilinear interpolation

Show your work

1. Where in the grid

   the EGM96 15-arc-minute grid, 0.25° between posts

   16.776°, -3.009° = 0.25° cell

2. Geoid height

   N = bilinear interpolation between the four surrounding posts

   at 16.776°, -3.009°, ±0.169 m from interpolation = 28.708 m

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Matches GeographicLib GeoidEval at its printed 0.1 mm on 2,010 points, including both poles. The grid itself departs from full EGM96 by up to 0.17 m (cubic); EGM96 is good to about 0.5-1 m worldwide.

When to use this: Use this whenever a GPS height has to become a height above mean sea level: the separation N between the ellipsoid and the geoid at a point is the number that connects them, and it reaches tens of meters. It is also what a drone, a survey, or an elevation product needs before its heights can be compared with published ones.

Limitations: This is the EGM96 global model: it is a smooth global surface, and a national or regional geoid model, such as a GEOID published for your country, is the one a survey is required to use. The gridded form here departs from full EGM96 by up to about 0.17 m, and the model itself has its own error against the true geoid.

Worked example: Timbuktu, Mali. Source: GeographicLib GeoidEval with egm96-15 (cubic): 28.7079 m. It is golden test vector v004, and every build checks the tool still gives its answer within its tolerance.

Review: Not yet independently reviewed by a geodesist.

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.1, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-23, fixed: Where the geoid is below the ellipsoid, the sentence said the geoid was a negative height above it ("-17.21 m above the ellipsoid"). It now says "17.21 m below the ellipsoid", and a point under the ground given reads as below the ground. The height converter's second example, a drone reporting 120 m above the ellipsoid over 250 m terrain, put the drone underground; it is replaced by a drone at 180 m above the ellipsoid over 40 m terrain, 117.539 m above the ground. Both tools are version 1.0.1; values are unchanged. Changelog
• 2026-09-18, changed: The first six tools passed the stable verification bar: a derivation, an independent worked example, invariant and differential tests, and at least 20 golden vectors each. Changelog

Checked against: 22 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• The Development of the Joint NASA GSFC and NIMA Geopotential Model EGM96, NASA/TP-1998-206861, Lemoine, F. G., et al., NASA Goddard Space Flight Center, NASA/TP-1998-206861. Geoid undulations relative to WGS 84.
• GeographicLib Geoid class and geoid data, Karney, C. F. F., GeographicLib, GeographicLib 2.x (egm96-15 grid, 2009-08-29). Geoid height interpolation: bilinear and 12-point cubic least-squares fit. Read free.

Terms

EGM96: Earth Gravitational Model 1996

A global gravity model whose geoid converts GPS ellipsoid heights to heights above mean sea level, to within about a meter. Source: EGM96 geopotential model and geoid