title: iPhone / iOS GPS Accuracy Statistical Confidence
body: <p>When you request a GPS <code>CLLocation</code> from iOS, it has a <a href="https://developer.apple.com/library/ios/documentation/CoreLocation/Reference/CLLocation_Class/index.html#//apple_ref/occ/instp/CLLocation/horizontalAccuracy" rel="noreferrer"><code>horizontalAccuracy</code></a> field, which is a radius in meters from the reported location where the user is likely to actually be.</p>
  <p>My question is this: <strong>With what statistical confidence (i.e. a percentage) can we say that the phone is within the &quot;horizontalAccuracy&quot; distance from the given location?</strong></p>
  <p>Some background on why I care:</p>
  <ol>
  <li><p>I'm working with data coming from both Android and iOS and I would like to know how confident I can be about any given piece of location data.</p>
  </li>
  <li><p>On Android, this data is explicit: 68%. From <a href="http://developer.android.com/reference/android/location/Location.html#getAccuracy()" rel="noreferrer">the docs</a>:</p>
  <blockquote>
  <p>We define accuracy as the radius of 68% confidence. In other words, if you draw a circle centered at this location's latitude and longitude, and with a radius equal to the accuracy, then there is a 68% probability that the true location is inside the circle.</p>
  </blockquote>
  </li>
  <li><p>In general, when dealing with GPS data, there are a number of standard error reporting methods including <code>RMS</code> (68%, likely the method Android uses), <code>2DRMS</code> (95%), <code>CEP</code> (50%), <code>Error Ellipse</code> (95%). You can read more about these methods <a href="https://en.wikipedia.org/wiki/Circular_error_probable" rel="noreferrer">here</a>.</p>
  </li>
  </ol>