GPS accuracy on iPhone

i’m building an app for a turist map of a town for iPhone.
The tourist walks around the town and his iPhone informs him when he arrives at a certain location.

What i’m trying to figure out is:
the client is shown as a red dot in google maps.

  1. how accurate i his gps location ( in meters).
  2. when the client starts to walk, how offen is his position refreshed? meaning if i walk 10 meters, will it show my movement a meter by meter?
    any answers or links will be apriciated
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