Accuracy

Warning

MilGPS is intended for navigation use only - applications that require precise positioning (better than consumer GPS accuracy of ~5m such as surveying) need special software and hardware designed for that purpose.

How accurate are locations given by the app?

When using the WGS84 datum, MilGPS will be as accurate as the GPS receiver in your phone. Typically 5m horizontal accuracy can be achieved with a clear view of the sky. The accuracy currently being reported by iOS is the accuracy that is displayed in the app.

When using a datum other than WGS84, the accuracy of displayed locations will depend on both the accuracy of the current location fix and the accuracy of the datum conversion. In general datum conversions introduce an additional 5-25m of inaccuracy. This depends both on the parameters used in the datum conversion and your location within the region covered by the datum. You can view the datum parameters used by MilGPS for a given datum by tapping the information button on the datum selection screen. This additional inaccuracy is not reflected in the accuracy displayed in the app.

MilGPS was developed for navigation purposes so in general the calculations inside the app give appropriate accuracy for navigation purposes, assuming consumer GPS accuracy.

I use an external bluetooth GPS with better accuracy, is the app more accurate now?

Unknown.

MilGPS does not interface directly with external bluetooth GPS receivers. Some apps, particularly those focused on GIS/surveying do. You should use one of those apps if you need a higher level of accuracy.

Instead, like most apps, MilGPS requests locations from iOS location services. The Core Location documentation states that "[t]he framework gathers data using all available components on the device, including the Wi-Fi, GPS, Bluetooth, magnetometer, barometer, and cellular hardware." It is not clear whether iOS location services passes through the data from an external bluetooth GPS untouched or whether it combines it with other measurements, possibly reducing its accuracy. It is our understanding that location services never reports better than 5m accuracy or updates more than once per second, even when external bluetooth units are paired.