App idea Map of free WiFi

Everyone likes free wifi. When we travel to a new place we should know about it. What if store free wifi ability on a map when you travel by your city. At the end of the day you will have approximated map of free wifi zones. Users can share their maps with each other to grow information about wifi in specific place. I don't know is it possible to do in background on iOS, but guess quite possible on windows phone and android. Also integrate this service on apple maps and google maps.