It was concluded that the performance of Assisted GPS on the iPhone at outdoor locations was substantially poorer than that achieved using a consumer-grade GPS receiver. The positional error for Assisted GPS was quite a bit larger compared to autonomous GPS. The average RMSE value for ten 20-minute tests was 9 meters horizontal and 10.6 meters vertical, several times larger than those for the consumer-grade GPS receiver. This is because the concessions that are made in the design of the Assisted GPS hardware on the iPhone, including antenna, power and other considerations.
Zandbergen, P.A. Accuracy of iPhone Locations: A Comparison of Assisted GPS, Wifi and Cellular Positioning, Transactions in GIS, 2009, 13(s1): 5-26, (Department of Geography: University of New Mexico).