How Accurate Is an iPhone GPS?by Hans Fredrick
The GPS on an iPhone is an assisted GPS system; it uses other reference sources in addition to the signal generated by the Global Positioning System satellites to pinpoint the exact location of the device. The more signal sources the iPhone can access for this purpose, the more accurate it becomes. In optimal conditions, the iPhone GPS is accurate to within a few meters.
The longer you keep an application that uses the GPS feature open on an iPhone, the more your accuracy increases. The more satellite signals the iPhone locks onto, the more accurate the results are. When using the location services on the phone, the GPS continually tries to pick up additional signals. If it succeeds, the accuracy of the results improves as a result.
The iPhone locks onto a location more quickly and with more accuracy if you allow it to connect to as many signal sources as possible. Enable both cellular service and Wi-Fi on the phone and connect to a Wi-Fi signal if the phone does not do so automatically to increase the accuracy of the iPhone's position.
Date and Time
It is critical that your iPhone is set to the correct date and time if you want accurate and quick GPS readings. When the time and date are set correctly, the GPS in the iPhone predicts where the satellite signal originates. This allows it to lock on to more satellites and to find those signals more quickly. Accuracy to within 10 meters can be expected as long as the phone locks on to at least three satellites.
Beginning with the iPhone 4S, Apple also uses Russia's GLONASS system to improve the accuracy and response time of the phone's location services. The Global Navigation Satellite System is by itself not as precise as GPS, but by using GLONASS signals alongside the other signal sources, the iPhone becomes more accurate. This service allows the iPhone to be accurate within 10 meters even in areas where GPS coverage is not strong, because these areas are sometimes better reached by the GLONASS system.