What is geoid undulation (N)?

Geoid undulation (N) is the vertical distance between the WGS84 reference ellipsoid and the geoid (approximation of Mean Sea Level). It varies from about -106 m in the Indian Ocean to +85 m near New Guinea.

Why is my GPS elevation different from map elevation?

GPS receivers natively compute ellipsoidal height (h), measured from the mathematical WGS84 ellipsoid. Maps typically show orthometric height (H), measured from Mean Sea Level (the geoid). The difference is the geoid undulation: h = H + N.

How accurate is this calculator?

This tool uses a degree-12 truncation of the EGM96 spherical harmonic model with 1-degree grid resolution and bilinear interpolation. Typical accuracy is within 5-10 m of the full EGM96 model. For survey-grade work, use the NGA's full EGM2008 model or local geoid models.

Geoid Undulation Calculator (EGM96)

Convert between GPS ellipsoidal height (h) and orthometric elevation above Mean Sea Level (H) using the EGM96 geoid undulation model (N). This tool runs entirely in your browser with no server calls.

Topographic Surface (ground)

Geoid (Mean Sea Level)

WGS84 Ellipsoid

h (ellipsoidal)

H (orthometric)

N (geoid)

h = H + N

🌐 Calculate Geoid Undulation

Latitude (decimal degrees)

Longitude (decimal degrees)

Loading EGM96 grid...

Geoid Undulation (N) —

Ellipsoidal Height (h) —

Orthometric Height (H) —

Formula Used —

Location —

⚠️ Accuracy Statement: This calculator uses a degree-12 truncation of the EGM96 spherical harmonic model with 1° grid spacing and bilinear interpolation. Typical accuracy is ±5–10 m relative to the full EGM96 model. For survey-grade work, use the NGS GEOID tool (USA) or the International Service for the Geoid with the full EGM2008 model at 2.5′ resolution.

Understanding Ellipsoidal vs. Orthometric Height

When a GPS receiver reports your altitude, it gives you the ellipsoidal height (h) — your distance above the mathematically smooth WGS84 ellipsoid. This is not the same as your elevation above sea level.

Maps and engineering projects use orthometric height (H), which is your elevation above the geoid — an irregular surface shaped by Earth's gravity field that approximates Mean Sea Level.

h = H + N
h = ellipsoidal height (GPS) | H = orthometric height (MSL) | N = geoid undulation

The geoid undulation (N) is the height of the geoid above (positive) or below (negative) the ellipsoid. It ranges from about −106 m in the Indian Ocean to +85 m near New Guinea. This calculator uses the EGM96 (Earth Gravitational Model 1996) to estimate N at any location on Earth.

EGM96 vs. EGM2008

Property	EGM96	EGM2008
Max degree/order	360	2190
Spatial resolution	~55 km	~9 km
Global RMS accuracy	±0.5 m	±0.1 m
Data size (full grid)	~2 MB	~700 MB
Publisher	NGA (1996)	NGA (2008)

Geoid vs Ellipsoid Explained → Elevation Finder → Ellipsoid Comparison → Map Viewer →