# Precision/Margin of Error (in meters) of iPhone/iPad Locations, Precise Location Off vs. On?

I am a mobile app developer developing a pair of iOS apps that use device location for search, and I’m trying to put constraints on the dimensions necessary for a cells in a quadtree-based location mapping system. The minimum cell size should roughly coincide with the margin of error for an iPhone or iPad with “use precise location” turned off.

Yes, I realize this likely varies by iPhone and iPad model, so if you can answer this question, please let me know which model(s) the statistics are for. I am interested in finding this information about models from the last 5 years or so. And I’m mostly interested in the margin of error for when precise location is turned off.

Does anyone know or know where I can find data on the precision of iPhone/iPad locations (i.e. a margin of error, as a range, in meters), for both precise location turned on and precise location turned off?

• So if you stand on a street corner, what does it give for your location with it on and off? practical research.. Jan 10 at 16:50
• This will depend on whether you're in the middle of nowhere; in an urban environment; indoors or outdoors. Whether the device has a cell signal, how fast you're moving… The map will give an estimated 'circle of precision' when you access it. See also apple.stackexchange.com/questions/238484/… Jan 10 at 17:27
• @Allan I think you've linked to the wrong thing Jan 10 at 18:37
• Thanks @Ezekiel. See this question/answer Jan 10 at 21:05
• I sure hope someone has some practical information or finds a write up on the web. This is actually something interesting to me - to figure of Apple is randomizing the error (which I would bet a small amount of money on) or it's somehow trackable via GPS and cell tower signal Trilateration.
– bmike
Jan 11 at 3:21

From a programmer's perspective, disabling the "Precise Location" feature is actually known as "reducedAccuracy". When in this mode, the app developer should only expect 1 to 20 kilometer horizontal accuracy. In addition, location updates will be delayed - so location data can be quite old (i.e. 15-20 minutes delayed).

I see in the comments that there is speculation that Apple applies a randomised error to the actual, precise location. That is not really the case. Instead Apple has mapped the globe into a number of cells, and when in reducedAccuracy mode, iOS tells the app only the center of the cell that the user is currently in.

The reason for this feature is that it has allowed Apple to create cells of varying size at locations around the world. This means that with reducedAccuracy an app would most likely be able to tell which (larger) city the user is currently in.

Had Apple just introduced a random 20 km error, or just mapped the globe into fixed sized cells, an app might mistake you for being in a completely different city.

• Thank you for your answer @jksoegaard, this is exactly the kind of info I am looking for. Is this data available in the Apple Developer docs somewhere? When you say "1-20km" for accuracy, are you referring to the range in the size of cells that they are using to map the spherical (well, as modeled) surface of the earth? Do you know which algorithm they use to accomplish this? Simple geohashing or quadtree-based? Do they use z-ordering or Hilbert Curve to represent 2D areas on the sphere as an ordered, 1D list of IDs? Do they use Google S2? Or is it just an arbitrary, proprietary mapping? Jan 16 at 14:55
• How would it matter to you which algorithm they use? - It is completely transparent to the developer. You can use any algorithm you want as an app developer for your own mapping purposes. Jan 16 at 15:43
• You wrote "Apple has mapped the globe into a number of cells". Therefore I took that at face value to mean that it was not up to the developer, but instead a mapping created by Apple themselves. If this is the case, as your answer implies, I am interested in what method they are using, because that would determine the dimensions of the cells (geohash = rectangular, while quadtree = square) and the way in which they are organized. Jan 16 at 18:09
• That mapping is created by Apple. The details of Apple's mapping is not exposed to the app. It does not mean that you cannot create your own mapping in your own app. Jan 16 at 22:25
• I am not interested in creating my own mapping, for any reason. I am interested in what Apple's mapping is, because your original post made it seem that the accuracy of user device locations is based on Apple's mapping when in `reducedAccuracy` mode. The entire point of this question is to get a sense of what this `reducedAccuracy` is, in concrete dimensions (or within some margin of error), and—if it changes geographically, as you've implied—how and where it does so. Jan 18 at 18:44