GPS is accurate-but it's not perfect. It doesn't work indoors; a tree or large bush can block the signal. What matters are the antenna in the receiver (both the size and where it's positioned), the number of channels available for tracking the GPS satellites and whether the device uses any additional systems to increase accuracy or to speed up getting a fix on the satellites.
Almost all devices sold today can track up to 12 satellites at once, while newer devices track up to 18 (the specifications may say 12-channel or 18-channel receiver). Your GPS receiver only needs to lock onto three or four satellites. However, it requires three or four strong signals, which will change as you travel and the satellites move. Tracking more satellites thus lets the device switch to the best signal more quickly (although it doesn't increase accuracy directly). As there are only 11 satellites in the western hemisphere and often only five to six will be in view at once, a 12-channel device can track all available signals.
|328 ft.||Accuracy of the original GPS system, artificially degraded by the Selective Availability (SA) program.|
|49 ft.||Typical GPS position accuracy without SA.|
|24 ft.||Worst-case WAAS position accuracy|
|10-16 ft.||Typical differential GPS (DGPS) position accuracy.|
|Under 7 ft.||Typical WAAS position accuracy.|
Table: GPS accuracy according to GPS manufacturer Garmin
The Wide Area Augmentation System (WAAS) does give you a more accurate position because it corrects the information from the satellites to account for errors in the orbit, ionospheric disturbance and other problems. There are other differential GPS systems based on radio transmissions or proprietary networks that also improve accuracy, but WAAS and the European equivalent EGNOS (currently still under test) are the standard. GPS receivers use some of their channels to track the WAAS satellites that broadcast the corrections, so 18 channels are useful for this. Originally implemented by the FAA for aviation, WAAS is becoming common on GPS devices.
Assisted GPS (AGPS) is used in smart phones with GPS to help the device find the satellites more quickly when you first turn it on. You download a tracking file at regular intervals so the device knows where to look for the satellites based on the time of day and the network cell you're in. This also improves battery life as scanning the whole sky for satellites takes more power than tracking a satellite on which the device already has a "fix". AGPS doesn't increase GPS accuracy. but it can mean that you get your location as you drive out of an underground parking lot rather than when you're a minute down the road and have already taken a wrong turn.