In practice, GPS accurracy and time to get the first fix are determined by the following factors:
- Satellite geometry: How many satellites are in the visible sky and how they are relatively located in the sky.
- Obstructions: Forest or other kind of obstacles affecting signal reception.
- Multipath effect: Tall buildings or canynons where signal bounces given wrong readings
- Receiver Chipset: Newer chipsets are able to use more satellites to compute location. They are also able to use Augmentation systems (EGNOS/WAAS) and assisted data (AGPS)
- Receiver antenna: Antenna size and geometry.
Other factors such as Cell tower triangulation and WiFi are different location systems, Smartphones use these systems together with GPS data to improve the location estimation in CERTAIN areas.
What does that means? A good GPS device with a modern chip under ideal conditions (open area, no tall buildings, good satellite signal) argues that you can expect aprox 3m accurracy 50% of the time. I'm not that optimistic and I definetely estimate 10m.
In dense urban areas with tall buildings, it could be quite worse and unpredictable. Thats because navigation systems such as TomTom use "matching" algoritms to match the computed position with a preloaded map.
Hope this help.