Strong opinions, weakly held

How Apple uses crowd-sourcing to find your location

In Apple’s explanation of why they kept a log of locations where your phone had been, they also explain how they use crowd-sourcing to allow iPhones to rapidly ascertain their location:

The iPhone is not logging your location. Rather, it’s maintaining a database of Wi-Fi hotspots and cell towers around your current location, some of which may be located more than one hundred miles away from your iPhone, to help your iPhone rapidly and accurately calculate its location when requested. Calculating a phone’s location using just GPS satellite data can take up to several minutes. iPhone can reduce this time to just a few seconds by using Wi-Fi hotspot and cell tower data to quickly find GPS satellites, and even triangulate its location using just Wi-Fi hotspot and cell tower data when GPS is not available (such as indoors or in basements). These calculations are performed live on the iPhone using a crowd-sourced database of Wi-Fi hotspot and cell tower data that is generated by tens of millions of iPhones sending the geo-tagged locations of nearby Wi-Fi hotspots and cell towers in an anonymous and encrypted form to Apple.

This technology seems really, really cool to me. I knew that Apple had a database of wifi hotspots that they used to determine where an iPhone is located without using GPS, but I didn’t realize that they were building the database using data uploaded from those phones.

As far as the location log goes, some programmer created the log but never bothered to scrub or rotate it. Been there, done that.


  1. Apple is mis-using the word “crowd-sourcing” here.

    Wikipedia is crowd-sourced. OpenStreetMap is crowd-sourced. Other people adding tags to your Flickr photos is crowd-sourcing.

    What Apple is doing is having barely–informed users contribute data, contributions they’d probably not make if iOS offered more fine grained control (i.e. I want to download data, but don’t see the need to contribute) and if users understood what was going on a little bit better (which is why this made so much news in the first place—users DON’T understand).

  2. I think that’s where this rubs me a little the wrong way as well; I’m not sure I was notified that I’d be contributing to the DB. I’m guessing they captured moments when I was associated with a hotspot and using GPS, somehow?

    If I am at home, turn off my cell service entirely and disable GPS but ask for my location, my iPhone can still show my current location as my home pretty accurately. Impressive, yes, but I’m not sure I ever said it was ok for them to collect that sort of information.

    In other words, under what circumstances did I send them the geographical location of my home wifi hotspot?

  3. I think that’s basically it. You don’t even have to connect to the hotspot, I imagine. It logs the location (if GPS is working) and hotspot serial number whenever it sees a hotspot and then anonymously uploads them periodically.

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