RTK and Fix Types
When collecting survey data, the fix type tells you how accurate your position is. Mapit GIS reads the fix quality from the GNSS receiver (via the GGA sentence's quality field) and displays it in the GPS Info sheet, so you always know the quality of the data you are collecting.
What Is RTK?
Real-Time Kinematic (RTK) is a positioning technique that achieves centimetre-level accuracy by using correction data from a nearby reference station (base station). The reference station has a known, precisely surveyed position. By comparing its calculated position with its known position, it generates correction messages that are transmitted to your receiver (the rover) in real time.
RTK corrections can be delivered to the receiver via:
- Radio link - the receiver's own radio modem communicates directly with a base station
- NTRIP - corrections are streamed over the internet from a network of reference stations using the built-in Mapit NTRIP client (requires mobile data or Wi-Fi)
- Companion app - a manufacturer's app (e.g., Eos Tools Pro, Trimble Mobile Manager) manages the NTRIP connection and delivers the corrected position to Mapit GIS
On iOS, NMEA data must arrive over a TCP/IP connection. Bluetooth serial and companion app relay are not currently supported.
Fix Quality Levels
The GGA sentence contains a quality indicator field (field 6) that reports the fix type. Mapit GIS displays this in the GPS Info sheet and uses it to compute position accuracy.
| Quality Code | Fix Type | Typical Accuracy | Description |
|---|---|---|---|
| 0 | Invalid | - | No position fix available |
| 1 | GPS (SPS) | 2-5 m | Standard autonomous GPS, no corrections |
| 2 | DGPS | 0.5-2 m | Differential GPS - corrections from SBAS (WAAS, EGNOS) or a base station |
| 3 | PPS | < 0.3 m | Precise Positioning Service (military/authorised users) |
| 4 | RTK Fixed | 1-3 cm | Full carrier-phase solution resolved - highest precision |
| 5 | RTK Float | 10-30 cm | Carrier-phase ambiguities not fully resolved - converging to fixed |
| 6 | Dead Reckoning | Varies | Estimated position based on last known fix and motion sensors |
| 7 | Manual | - | Position entered manually |
| 8 | Simulation | - | Simulated position for testing |
For professional surveying, aim for RTK Fixed (quality 4). If you see RTK Float (quality 5), the solution is still converging - wait for it to transition to Fixed before collecting data. This typically takes 10-60 seconds depending on satellite visibility and baseline length.
SBAS Detection
When the receiver uses satellites from a Satellite-Based Augmentation System (SBAS) - such as WAAS (North America), EGNOS (Europe), GAGAN (India), or MSAS (Japan) - the app automatically detects the correction satellite and upgrades the fix quality to DGPS (quality 2) if it was previously reported as standalone GPS.
The SBAS satellite name is shown in the GPS Info sheet when active.
Fix Mode (2D vs. 3D)
In addition to fix quality, the GSA sentence reports the fix mode:
| Fix Mode | Meaning |
|---|---|
| 1 | No fix |
| 2 | 2D fix - latitude and longitude only (altitude unreliable) |
| 3 | 3D fix - latitude, longitude, and altitude |
A 2D fix requires at least 3 satellites; a 3D fix requires at least 4. For surveying work, a 3D fix is essential for reliable height data.
How Accuracy Is Computed
Mapit GIS computes the displayed accuracy from multiple sources, using the best available:
- Base accuracy = HDOP multiplied by a precision factor that depends on fix quality:
| Fix Quality | Precision Factor |
|---|---|
| GPS (SPS) | 5.1 |
| DGPS | 1.5 |
| PPS | 0.25 |
| RTK Fixed | 0.25 |
| RTK Float | 0.3 |
-
If the receiver provides HRMS (horizontal root mean square) via a GST or PNVGSDP sentence, and the HRMS value is better than the HDOP-based estimate, the HRMS value is used instead.
-
If the receiver provides a standalone RMS value that is better still, that value takes precedence.
This layered approach ensures the most accurate estimate is always displayed, regardless of which data the receiver provides.
RMS Multiplier
For applications requiring a specific confidence level, you can apply an RMS multiplier to the displayed accuracy. For example, a multiplier of 1.96 converts 1-sigma (68%) accuracy to 2-sigma (95%) accuracy.
Configure this in Settings > Survey Settings > RMS Multiplier.
Differential Correction Details
When differential corrections are active, the GPS Info sheet shows:
| Field | Description |
|---|---|
| Correction age | Seconds since the last correction was applied. A low value (< 5 s) indicates active corrections. |
| Correction station | The identifier of the reference station providing corrections. |
If the correction age exceeds 30 seconds, the corrections may be stale and accuracy will degrade. Check your NTRIP connection or radio link.