Most Effective way to check player is at GPS location?

Hi,

I want to add features in to my game that change dependent on the location your playing from, similar to how snapchat overlays work for certain citys,

I understand how to get the current location of the device from 'Input.location.lastData;' I'm just wondering if anyone had any advice on what would be the most effective way to check a player is within range of a location?

I have been trying to do it like this but its very hit and miss, and not very accurate at all (in fact I've never been able to leave the location and then register returning to it)

http://i.imgur.com/aahDqwE.png

http://i.imgur.com/MBdI9xW.png