Support
Welcome to the Undulation Calculator support page. Here you'll find answers to common questions, troubleshooting tips, and information about how to use the app.
Quick Start
To calculate geoid undulation at a location:
- Open the app and go to the Calculator tab
- Enter coordinates in one of these formats:
- Decimal degrees:
(48.8566, 2.3522)
- With cardinal directions:
48.8566°N, 2.3522°E
- Paste coordinates directly from Apple Maps or Google Maps
- The app will automatically calculate and display the geoid undulation value
- Optionally enter an altitude to see conversions between MSL, HAE, and AGL
Frequently Asked Questions
What is geoid undulation?
Geoid undulation (N) is the height difference between the geoid (Mean Sea Level) and the WGS84 ellipsoid used by GPS systems. GPS devices report altitude relative to the WGS84 ellipsoid, but many real-world applications need height above Mean Sea Level. The undulation varies by location, typically ranging from about -106 meters to +85 meters.
What coordinate formats are supported?
The app accepts coordinates in several formats:
(latitude, longitude) - e.g., (48.8566, 2.3522)latitude°N/S, longitude°E/W - e.g., 48.8566°N, 2.3522°E- You can paste coordinates directly from Apple Maps or Google Maps
Latitude must be between -90° and 90°. Longitude must be between -180° and 180°.
What do MSL, HAE, and AGL mean?
- MSL (Mean Sea Level): Height above the geoid (Mean Sea Level)
- HAE (Height Above Ellipsoid): GPS altitude relative to the WGS84 ellipsoid
- AGL (Above Ground Level): Height above the terrain at that location
The relationship is: HAE = MSL + Geoid Undulation and AGL = HAE - Terrain Elevation
What data sources does the app use?
- EGM96 (Earth Gravitational Model 1996) - Official geoid model at 15 arc-minute resolution for geoid undulation calculations
- ETOPO1 - Global terrain elevation model at 1 arc-minute resolution (~1.8 km) covering both land and ocean bathymetry
Both datasets are embedded in the app and all calculations are performed locally on your device.
How accurate are the calculations?
The accuracy depends on the data sources:
- Geoid undulation: Within ±1 meter using the EGM96 model (15 arc-minute resolution)
- Terrain elevation: Approximately ±10-20 meters using ETOPO1 data (1 arc-minute resolution, ~1.8 km)
The app uses bilinear interpolation for sub-grid accuracy between data points.
Does the app require internet connection?
No. Undulation Calculator works completely offline. All geodetic data is embedded in the app, and all calculations are performed locally on your device.
What units are supported?
The app supports both meters and feet for all elevation values. You can switch between units in the Settings section of the Calculator tab.
What does negative AGL mean?
A negative AGL (Above Ground Level) value means your entered altitude is below the terrain elevation at that location. This could occur if you're in a valley or if the terrain data indicates higher elevation than your input.
Using the Features
Calculator Tab
Enter coordinates to calculate geoid undulation and terrain elevation. Optionally enter an altitude to see conversions between MSL, HAE, and AGL reference systems.
Graphs Tab
Visualize geoid undulation and terrain elevation along longitude and latitude cross-sections. The graphs show:
- Geoid surface (MSL) in blue
- Terrain surface (HAE) in brown
- WGS84 ellipsoid reference line at 0
- Your current location marked with a red dot
Globe Tab
Explore a 3D visualization of Earth showing:
- Color-coded geoid undulation (blue = positive, red = negative)
- Actual terrain geometry with realistic lighting
- Ocean bathymetry
Rotate the globe by dragging, pinch to zoom, and tap to select locations.
Troubleshooting
Coordinates not calculating
- Verify coordinates are in valid ranges: latitude -90° to 90°, longitude -180° to 180°
- Check the coordinate format - use
(lat, lon) or paste from maps
- Ensure there are no extra spaces or invalid characters
Globe not loading
- The initial globe generation can take 30-60 seconds - please wait
- The globe requires significant processing - make sure your device has sufficient memory
- Try closing and reopening the app if the globe doesn't appear
Slow performance
- Globe generation is computationally intensive and may take time
- Large profile graphs (±5° range) sample many points and may be slow
- Calculations are performed on-device for privacy - there's no server-side processing
Technical Information
Data Models
- EGM96: 721 rows × 1440 columns (15 arc-minute grid)
- ETOPO1: 10,801 rows × 21,601 columns (1 arc-minute grid)
Interpolation
The app uses bilinear interpolation to calculate values between grid points for sub-grid accuracy.
Coordinate Systems
- Input coordinates: Decimal degrees (WGS84)
- All calculations: WGS84 reference frame
- Geoid model: EGM96 (WGS84 compatible)
Test Coordinates
Try these known locations to verify the app is working correctly:
- Paris, France:
(48.8566, 2.3522) - Undulation ≈ 47.5 m
- New York, USA:
(40.7128, -74.0060) - Undulation ≈ -30.0 m
- Tokyo, Japan:
(35.6762, 139.6503) - Undulation ≈ 39.0 m
- Sydney, Australia:
(-33.8688, 151.2093) - Undulation ≈ 14.0 m