title: Why does iOS show me a "home" location when I'm abroad without data roaming?
body: <p>I'm working on an iOS app which uses <code>Core Location</code>.</p>
  
  <p>I'm usually based in the UK but have been in the US for the past week. When the app can't get a location fix, or doesn't have data service (I've turned roaming data off) it confidently reports its location as Piccadilly Square in London, but when I'm connected to a known WiFi point it corrects itself to the other side of the Atlantic.</p>
  
  <p>Why is this, and how can I check for erroneous location information (e.g. confident reports of being in London) and filter them out?</p>