Height and Geoid Models
GNSS receivers measure height above a mathematical surface called the ellipsoid, but most real-world applications need height above mean sea level (MSL). The difference between these two surfaces is called the geoid separation, and it varies by location. Mapit GIS provides built-in geoid models to convert between the two, plus support for custom and externally-provided geoid corrections.
Ellipsoidal vs. Orthometric Height
| Term | Definition |
|---|---|
| Ellipsoidal height (h) | Height above the WGS 84 reference ellipsoid - what the GNSS receiver measures directly |
| Orthometric height (H) | Height above mean sea level (the geoid) - what you see on topographic maps and benchmarks |
| Geoid separation (N) | The difference between the ellipsoid and the geoid at a given location |
The relationship is:
H = h - N
Orthometric height = Ellipsoidal height - Geoid separation
The geoid separation can range from approximately -100 m to +85 m depending on your location on Earth.
- Android
- iOS
Enabling Geoid Correction
- Open Settings > Survey Settings.
- Toggle Apply geoid height correction on.
- Select a Geoid model from the dropdown.
When enabled, the app applies the geoid correction to all displayed heights and recorded point elevations.
Available Geoid Models
Mapit GIS includes a range of geoid models, from a global embedded model to high-accuracy national grids. The geoid correction is computed using native RTKLib code via JNI for maximum performance.
Global Models
| Model | Coverage | Resolution | Notes |
|---|---|---|---|
| Embedded EGM96 | Worldwide | 1 x 1 degree | Built into the app - no download required. Suitable for general use. |
| EGM2008 | Worldwide | 2.5 x 2.5 arc-minutes | Higher accuracy global model. Requires a grid file download. |
National / Regional Models
| Model | Country | Notes |
|---|---|---|
| Geoid 12B | USA | US national geoid model |
| HT 2.0 | Canada | Canadian geoid model |
| OSGM15 | Great Britain | Ordnance Survey geoid model for Great Britain |
| RAF09 | France | Older French geoid model |
| RAF18 | France | Current French geoid model |
| CHGEO04 | Switzerland | Swiss national geoid model |
| PLGEO11 | Poland | Polish geoid model |
| PL-EVRF2007-NH | Poland | Polish model in EVRF2007 reference |
| DVRM05 | Slovakia | Slovak national geoid model |
| Geoid Grid Slovenia | Slovenia | Slovenian national geoid model |
Special Options
| Option | Description |
|---|---|
| Custom geoid (.bin) | Load a custom geoid grid file in NGS little-endian binary format |
| External software | Use the geoid correction provided by a third-party companion app (e.g., Eos Tools Pro, Trimble Mobile Manager). The app reads the MSL height directly from the location bundle. |
For highest vertical accuracy, use the national geoid model for your country if one is available. Global models like EGM2008 are accurate to approximately 10-20 cm, while national models are typically accurate to 1-3 cm.
How Geoid Correction Is Applied
The correction pipeline depends on the position source:
External GNSS (Bluetooth / TCP)
- The NMEA GGA sentence provides both MSL altitude and geoid separation.
- The app computes ellipsoidal height as:
altitude = MSL + geoid_separation. - If geoid correction is enabled, the selected geoid model is used to compute the orthometric height from the ellipsoidal height.
Third-Party Companion Apps
When the companion app provides both mslHeight and undulation in the location extras:
- If the geoid model is set to External software: the MSL height from the companion app is used directly (the app trusts the companion's correction).
- Otherwise: the app converts the MSL height back to ellipsoidal and applies its own geoid model.
Antenna Height
An antenna height offset accounts for the physical distance between the GNSS antenna's phase centre and the ground point being measured. This is critical for accurate elevation data.
Configuration
- Open Settings > Survey Settings.
- Set the Antenna height value in metres (default: 1.30 m).
The antenna height is displayed in the settings as: "Antenna height: {value} m".
How It Is Applied
The antenna height is subtracted from the GNSS-reported height for every recorded point:
Recorded height = GNSS height - Antenna height
This means the recorded height represents the ground level at the measurement point, not the antenna position.
Always measure and update the antenna height when changing your survey setup (e.g., switching poles, changing the receiver mounting). An incorrect antenna height directly affects all recorded elevations.
Geoid model selection and custom geoid grids are not yet available on iOS. The app currently uses the geoid separation value provided by the receiver in the NMEA GGA sentence. Full geoid model support is planned for a future update.
Antenna Height
You can configure an antenna height offset in Settings > Survey Settings. This value is subtracted from the GNSS-reported height for every recorded point, so the recorded elevation represents the ground level at the measurement point.