Gps Problem

Hi all, i have problem with my gps, i have tried fast gps, changing gps.conf and i cant fix it. My problem is that my gps works fine on road, when i am in a road takes my position very well but when i go inside a field or a park, my gps thinks that i am in the nearest road ( in the edge of the park) and i cant catch pokemon inside of a park xD.
I have tried a lot of things but i cant solve it, it just than my gps is always in a road, how can i solve it?
Thanks and sorry for my bad english