Geoid Calculator

GPS Altitude & Geoid Tool. Calculate geoid undulation at any point on Earth with precision and ease.

Undulation Calculator is a powerful tool for geodesy professionals, surveyors, pilots, engineers, and anyone working with GPS coordinates and elevation data. Calculate geoid undulation (the difference between Mean Sea Level and the WGS84 ellipsoid) at any point on Earth with precision and ease.

Calculate the geoid undulation (N value) at any location using the official EGM96 (Earth Gravitational Model 1996) dataset. Understand the relationship between the geoid (Mean Sea Level) and the WGS84 ellipsoid used by GPS systems.

Get precise terrain elevation information using the ETOPO1 global relief model with 1 arc-minute resolution (~1.8 km). Includes both land elevations and ocean bathymetry data.

Easily convert between three altitude reference systems:

• MSL (Mean Sea Level) - Height above the geoid
• HAE (Height Above Ellipsoid) - GPS altitude relative to WGS84
• AGL (Above Ground Level) - Height above terrain

Explore a stunning 3D visualization of Earth showing:

• Color-coded geoid undulation map (blue for positive, red for negative)
• Accurate terrain geometry with realistic lighting
• Ocean bathymetry visualization
• Rotate, zoom, and explore the globe interactively

Visualize geoid undulation and terrain elevation along longitude and latitude cross-sections:

• Interactive charts showing geoid (MSL) and terrain (HAE) surfaces
• Current location marker
• WGS84 ellipsoid reference line
• Easy-to-read axis labels with unit conversions

Enter coordinates in multiple formats:

• Decimal degrees: (48.8566, 2.3522)
• Degrees with cardinal directions: 48.8566°N, 2.3522°E
• Paste directly from Apple Maps or Google Maps
• Automatic coordinate parsing and validation

Switch between meters and feet for all elevation values, with precision appropriate to each measurement type.

Uses official geodetic datasets:

• EGM96 geoid model (15 arc-minute resolution)
• ETOPO1 terrain model (1 arc-minute resolution)
• Bilinear interpolation for sub-grid accuracy

Perfect For:

• Surveyors and geodesy professionals
• Pilots and aviation professionals
• Engineers working with GPS/GNSS data
• Students studying geodesy and cartography
• Anyone needing to understand the relationship between GPS altitude and sea level

Technical Details:

• EGM96: Earth Gravitational Model 1996, the international standard for geoid undulation
• ETOPO1: Global 1 arc-minute relief model covering land and ocean
• WGS84: World Geodetic System 1984, the standard GPS coordinate system
• All calculations performed locally on-device for privacy and offline use

GPS devices report altitude relative to the WGS84 ellipsoid (HAE), but most real-world applications need height above Mean Sea Level (MSL). The difference between these two surfaces is called "geoid undulation" and varies from location to location, ranging from about -106 meters (Indian Ocean) to +85 meters (Iceland). Understanding this relationship is crucial for accurate elevation measurements.