Geoid Models
- Android
- iOS
Geoid model selection on Android is part of Survey Settings. See the Orthometric height and Geoid model sections there.
The Geoid Models screen lets you download, activate, and manage geoid grid files used to convert ellipsoidal GPS altitude to orthometric (mean sea level) altitude. It is only reachable when Orthometric Height is toggled on — navigate to Settings → Survey → Orthometric Height, then tap the Geoid model row.
Orthometric Height correction requires a Mapit Pro Plus subscription. Free and Pro users see the toggle but tapping it opens the Pro+ paywall.
The Orthometric Height feature applies corrections only when data arrives from an external GNSS receiver connected via Bluetooth (MFi), Bluetooth Low Energy, or TCP/IP NMEA.
The iPhone's built-in GPS already reports mean sea level altitude internally via CoreLocation — no app-side correction is needed or applied. Enabling Orthometric Height has no effect on the internal GPS; the altitude you see in Mapit GIS will be identical whether the toggle is on or off when using the iPhone's own location hardware.
How it works
External NMEA receivers report two altitude-related fields in every GGA sentence:
- MSL altitude — height above the geoid approximation embedded in the receiver
- Geoid separation — the offset between that embedded approximation and the WGS 84 ellipsoid at the current position
When Orthometric Height is enabled, Mapit GIS:
- Reconstructs the ellipsoidal height:
h = MSL + geoid_separation - Evaluates the selected geoid grid at the current latitude and longitude to get the precise separation
N - Stores the corrected orthometric height:
H = h − N
This overrides the receiver's own (often low-resolution) embedded geoid with whichever grid you have selected — allowing you to use a national high-accuracy model rather than the basic approximation hardwired into the receiver firmware.
The Geoid Models picker
The picker is divided into two sections.
Embedded
| Model | Coverage | Notes |
|---|---|---|
| EGM96 | Worldwide | Built into the app. No download required. Automatically selected when you first enable Orthometric Height. |
The Embedded section always shows a green checkmark when EGM96 is the active model.
Regional & Worldwide Grids
Eleven additional grids can be downloaded directly to the device. Each row shows:
- Grid name and country/region
- A download button when the grid is not yet on the device
- A Select button on grids that are downloaded but not active
- A green checkmark on the currently active grid
- A left-swipe delete option on downloaded (non-active) grids
| Grid | Coverage | Notes |
|---|---|---|
| EGM2008 (2.5′ worldwide) | Worldwide | High-resolution global model; preferred over EGM96 when no national grid is available |
| OSGM15 | United Kingdom | Ordnance Survey geoid for Great Britain and Northern Ireland |
| GEOID12B | United States | NOAA/NGS national geoid model for the contiguous US, Alaska, Hawaii |
| HT 2.0 | Canada | NRCan Canadian geodetic vertical datum |
| PLGEO11 | Poland | Polish national geoid model |
| PL-EVRF2007-NH | Poland | Polish geoid in the EVRF2007 reference frame |
| RAF18 | France | Current IGN French geoid model |
| RAF09 | France | Older IGN French geoid model (superseded by RAF18) |
| CHGEO04 | Switzerland | Swiss national geoid model |
| PT08 | Portugal | Portuguese national geoid model |
| GeoID-SLO | Slovenia | Slovenian national geoid model |
For any survey that will be compared against national benchmarks or shared with clients expecting orthometric heights, choose the national grid for your country rather than the global EGM2008 model.
Downloading a grid
- Locate the grid you want in the Regional & Worldwide Grids section.
- Tap the download button on that row.
- The download progress is shown inline. Grid files range from a few megabytes (national) to larger files for EGM2008 — a Wi-Fi connection is recommended.
- Once downloaded, a Select button replaces the download button.
Grids are stored locally on the device and only need to be downloaded once. They remain available offline.
Activating a grid
Tap the Select button on any downloaded grid to make it the active model. The row gains a green checkmark and the active model name is reflected as the trailing label on the Geoid model row in Survey Settings.
Deleting a grid
To free up storage, swipe left on any downloaded grid that is not currently active and tap Delete.
You cannot delete the active grid directly. To delete it, first activate a different grid (or the Embedded EGM96), then swipe-delete the previously active one.
If you delete the active grid while Orthometric Height is enabled, the app automatically falls back to the embedded EGM96.
Behaviour at a glance
| GPS source | Toggle | Altitude stored |
|---|---|---|
| iPhone internal GPS | Off | MSL (CoreLocation already corrects internally — no change) |
| iPhone internal GPS | On | Same as Off — toggle has no effect on the built-in GPS |
| External NMEA receiver | Off | GGA field-9 MSL altitude (receiver's own embedded geoid) |
| External NMEA receiver | On — EGM96 (embedded) | (MSL + sep) − EGM96(lat, lon) — re-computed with the bundled grid |
| External NMEA receiver | On — regional grid (e.g. OSGM15) | (MSL + sep) − grid(lat, lon) — overrides receiver firmware with the high-accuracy national model |
Related
- Survey Settings — enable Orthometric Height and navigate to this screen
- Height and Geoid Models — background on ellipsoidal vs. orthometric heights
- GPS / GNSS Settings — configure your external receiver connection
- Mapit Pro Plus — subscription required for this feature