We’re implementing a feature in our app that allows a certified MFi GPS device to act as the default location source for user positioning, replacing the internal GPS when connected. However, we’re noticing a significant discrepancy between the accuracy values reported by iOS and those directly available from the GPS device.
Here’s the issue:
When the MFi GPS is in use, it transmits standard NMEA messages (GGA, GSA, RMC, etc.) to the iOS device (providing HDOP and VDOP). However, the accuracy reported by iOS for this “location-system” seems to be less precise than what the GPS device itself reports.
For example, here are the readings we observe:
Location from iOS device (systemLocation):
Position: <+41.4, +1.8> +/- 5.00m (speed 0.05 m/s / course 329.40)
Timestamp: 6/11/24, 12:01:50 Central European Standard Time
Horizontal Accuracy (from systemLocation): 5.0 meters (16 ft)
Vertical Accuracy (from systemLocation): 9.5 meters (31 ft)
Location from GPS device directly:
Horizontal Accuracy: 9.1 ft
Vertical Accuracy: 10.3 ft
It’s evident that the accuracy values displayed by iOS differ from the values available from the GPS device, especially in terms of horizontal and vertical accuracy.
Question: Is there a known reason for this discrepancy in accuracy values? Is there a way to obtain the GPS device’s native accuracy values in iOS, or is iOS applying additional filtering or adjustments that might explain this difference? If additional filtering is applied can be disabled?
Any insights would be greatly appreciated, as accurate location reporting is critical for our app’s functionality.
