One of the most exciting new features since iOS5 is the ability to setup reminders - and even better - the ability to setup geofenced reminders based on when you arrive or leave a location.

I have two key questions about how these work in practice and I was wondering if anybody could answer them based on their experiences.

    1.) If I am sitting in location A but plan to leave and setup a reminder that will go off when I arrive back in location A - will that work?

i.e. if you are in your office and are going to be leaving for a meeting and want to remember to do something when you get back can you setup that alert before you leave or will it just go off immediately because you are already there?

    2.) I live in Manhattan and my home address (stored in my contact information) is not what my iPhone thinks my location is when I am sitting in my apartment. My iPhone thinks I am three buildings down and across the street. Will geofenced reminders work if I tell them to remind me when I get "Home" even though "Home" is not exactly where iPhone GPS thinks I live?

NB: My one-off test of this shows that it does not correctly identify me as being home. It looks like I would have to tell it to remind me when I get to the incorrect address that iPhone thinks is home. I would think it would have some radius to allow for inaccuracies of GPS - would be best if you could SET this radius!

  • GPS is not that inaccurate. If you're shown down the road, I'd sooner blame WiFi/Cell Tower triangulation rather than GPS. (And, since GPS requires clear view of the sky and you're presumably indoors, I would doubly blame WiFi/Cell Tower triangulation.) Oct 20, 2011 at 0:52
  • @VxJasonxV Agreed - I guess I mean general iPhone "geolocation" and not GPS necessarily. In Manhattan it often is hard to have the clear view of the sky!
    – Jish
    Oct 20, 2011 at 1:56
  • Touché ;). (4 more to go...) Oct 21, 2011 at 6:14
  • GPS can be horribly inaccurate. Multipath errors, low signal, intentional errors added (which have been turned off for years, but the capability is there for error to be induced). Also, in some areas, the map data in the tiles isn't correctly aligned so even though your phone might have great GPS coordinates - it doesn't place you on the map correctly.
    – bmike
    Oct 28, 2011 at 14:32

2 Answers 2


I'll split your question into two main areas since one is easily answerable, the other isn't without inside knowledge of how location services work within iOS.

  • How to make a reminder for when you re-arrive at the place you sit now?
  • What is the special sauce and all the ways I can tweak location in iOS?

To make a reminder, start by turning on "At a Location" and then choose when I arrive instead of the default when I leave. Sometimes I have poor location signal and the alarm may go off, but that seems to happen very rarely in practice. When that happens, I just set it to when you leave. That seems to be less sensitive to radius so when it goes off, I then remember to change it back to arrival.

The deeper question of how location really works and is implemented is not publicly disclosed. As an end user, you can't set radius or other parameters that affect the location engine. It is either off or on. You either have the radios on or off.

I would encourage you to get Xcode and play with some of the demonstration projects to learn how location works in the iOS simulator and perhaps you can gain a better understanding of how iOS presents this data to apps.


The Reminders app doesn't expose all the functionality of geofencing that the iOS 5 api allows. So a third party app could give you some more options about this.

There's a location manager object that the OS maintains, and developers interface with that to configure location services, and to be informed about things that have happened inside the core location framework.

One thing that the location manager can do is, it can be set up to monitor a list of regions. Each region has a center point and a radius. It has to be circular, but you can set how big a circle.

The other thing you can tell the location manager when registering a region to be watched is the "accuracy"--how far "past" the boundary of that region the user has to be before firing the "We're here!" message into the client code. That's useful to account for exactly what you're seeing. Where you are, your GPS accuracy is low, so it'd be good if the jitter introduced into the data by larger-grained positioning methods wasn't firing alerts willy nilly just because you happened to be in the ballpark of your region.

None of this configurability is exploited in the OS's reminders app--it just takes an address and builds a region around it. In my experience it's probably got a fairly large "accuracy" setting. An alert leaving my office fires maybe 1/8 mile down the road. An alert arriving at my home fires as I pull into my driveway. So maybe it's 100 meters plus or minus 75?

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