Inaccurate Maps location iPhone 4

Even on ground level in an open area I still end up 10-20 metres off my exact location,I noticed this when Im at particular landmarks and the phone shows me across the street.I have been testing this over the course of a few weeks at different locations but with the same results.
Before the OS4 update for my Iphone 4 it was working fine and gave very decent and accurate readings.So Im quite sure its nothing to do with the hardware.
Any opinions or solutions to this?
Anyone experiencing inaccuracy with their phones?
Iphone 4 iOS 4