I don’t think region monitoring will work well for such a small radius.
- The best accuracy with the GPS chip and
kCLLocationAccuracyBestForNavigationis often just 10 meters. - Apple says (in the Location & Maps PG) that the minimum distance for regions should be assumed to be 200m
- I’ve heard that region monitoring is using WiFi to get it’s position (which makes sense for power savings). WiFi accuracy is more like 20m-100m. I’m not sure how having another app using background location (i.e. using GPS) would affect this. Probably, the location manager would share information to make the accuracy better.
- Region monitoring can take 30 seconds to fire once inside a region, and a couple of minutes to fire after leaving a region (to prevent location glitches from triggering it).
- When region-monitoring was first introduced, they said that it would only work with 100m regions and anything smaller would be bumped up. This probably still happens.
- There’s a deprecated method
startMonitoringForRegion:desiredAccuracy:which allowed you to specify the distance past the region border to start generating notifications. Presumably this feature has been rolled intostartMonitoringForRegion:but is still there. A 10m region might end up with a 10m buffer. - If you want to do this, specify a larger region around where you want to monitor, and when the device wakes up in that region, start background location updates (GPS) and use
CLCircularRegion‘s-containsCoordinate:to trigger when the device is within 10m manually. This method is officially sanctioned by Apple (see WWDC 2013 Session 307).
From the CLCircularRegion docs:
Remember that the location manager does not generate notifications immediately upon crossing a region boundary. Instead, it applies time and distance criteria to ensure that the crossing was intended and should genuinely trigger a notification. So choose a center point and radius that are appropriate and give you enough time to alert the user.
From the Location & Maps PG:
Region events may not happen immediately after a region boundary is crossed. To prevent spurious notifications, iOS doesn’t deliver region notifications until certain threshold conditions are met. Specifically, the user’s location must cross the region boundary, move away from the boundary by a minimum distance, and remain at that minimum distance for at least 20 seconds before the notifications are reported.
The specific threshold distances are determined by the hardware and the location technologies that are currently available. For example, if Wi-Fi is disabled, region monitoring is significantly less accurate. However, for testing purposes, you can assume that the minimum distance is approximately 200 meters.
There’s further inside scoop from this post by Kevin McMahon, who asked the Core Location engineers about region monitoring at a lab at WWDC 2012. This info will have changed in the meantime, but the part about region categories is interesting. Here’s an edit:
Fine Region (0 – 150m)
– With the floor of 100m this category’s range is effectively 100-150m.
– For regions this size performance is heavily dependent on the location-related hardware
– The amount of time that it takes Core Location to detect and call the appropriate delegate method is roughly 2-3 minutes on average after the region boundary has been crossed.
– Some developers have figured out independently that smaller regions would see quicker callbacks and would cluster smaller regions to cover one large area to improve region crossing notifications.