Tracking multiple (20+) locations with iOS geofencing

Question

An iOS application uses the geofencing for notifying the user about predefined nearby locations. The application is allowed to miss some location (the user is not getting a notification about a nearby location), but it is desirable to keep the missing rate low.

One way to implement this would be to start monitoring for significant change locations with startMonitoringSignificantLocationChanges and each time the "location change" event is fired, look for locations within, let say, 500m radius of the reported location.

What worries me is the requirement to perform the query for the nearby regions each time the significant location change occurs and it impact on the battery.

The other way to do it would be to register the locations with startMonitoringForRegion but Apple has put a (reasonable) limitation on the number of simultaneously tracked regions which is 20 and we have significantly more than 20 locations. So some sort of dynamic updating of the tracked regions is required but I am still unsure what is the best way to do it.

Any ideas on how can it be done so that it keeps the battery consumption low but also has the low missing rate for locations?

Answer 1

Since there was not much activity on the question I will describe how we are currently solving this problem.

We tied the reloading of the new regions to significant location change (SLC) events. When an SLC takes place, we check for 20 neighbouring regions that should be "geofenced". To find the 20 closest regions we are simply approximating 1'' of the latitude and longitude according to the following formulae:

Latitude: 1 deg = 110.54 km

Longitude: 1 deg = 111.320 * cos(latitude) km

and just check the bounding square of the current position of the device for the centers of the monitored regions (see: Simple calculations for working with lat/lon + km distance?)

So, for example, if (10N,10E) is the current location of the device we start with the bounding square with vertices at (10-1',10-1'), (X-10',10+1'), (10+1',10+1'), (10+1',10-1') (at latitude (10N,10E) one latitude/longitude minute approximates 1,85 km).

If there are 20 (or almost 20) - we register them for the geofencing and wait for the next SCL. If less/more, just increase/decrease the size of the bounding rectangle and repeat the search.

You can tweak this search algorithm for a better performance, but the one described here will already do the job.

Answer 2

You could reserve a location for a "meta-geofence" encompassing all the currently monitored locations. When the user leaves this geofence, the app will be notified. Then the app can update itself and stop tracking the farthest areas and start tracking new areas in the vicinity.

Answer 3

I thought I would add another option for using more than 20 Geofences in your app. This way has been working well in our app for a long time now and uses CLLocation methods that are built-in.

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations {
    if (locations.count > 0) {
        CLLocation *location = locations[0];

        NSMutableArray *sortedFences = [[NSMutableArray alloc] init];

        // add distance to each fence to be sorted
        for (GeofenceObject *geofence in enabledFences) {
            // create a CLLocation object from my custom object
            CLLocationCoordinate2D coordinate = CLLocationCoordinate2DMake(geofence.latitude, geofence.longitude);
            CLLocation *fenceLocation = [[CLLocation alloc] initWithLatitude:coordinate.latitude longitude:coordinate.longitude];
            // calculate distance from current location
            CLLocationDistance distance = [location distanceFromLocation:fenceLocation];
            // save distance so we can filter array later
            geofence.distance = distance;
            [sortedFences addObject:geofence];
        }

        // sort our array of geofences by distance and add we can add the first 20

        NSSortDescriptor *sortByName = [NSSortDescriptor sortDescriptorWithKey:@"distance" ascending:YES];
        NSArray *sortDescriptors = [NSArray arrayWithObject:sortByName];
        NSArray *sortedArray = [sortedFences sortedArrayUsingDescriptors:sortDescriptors];

       // should only use array of 20, but I was using hardcoded count to exit

        for (GeofenceObject *geofence in sortedArray) {
            CLLocationCoordinate2D coordinate = CLLocationCoordinate2DMake(geofence.latitude, geofence.longitude);
            CLLocationDistance radius = geofence.radius;
            NSString *ident = geofence.geofenceId;

            CLCircularRegion *fenceRegion = [[CLCircularRegion alloc] initWithCenter:coordinate radius:radius identifier:ident];
            fenceRegion.notifyOnEntry = geofence.entry;
            fenceRegion.notifyOnExit = geofence.exit;
            [locationController.locationManager startMonitoringForRegion:fenceRegion];
        }
    }
}

Hopefully this will help someone or steer them on the right path.

Answer 4

If you are concerned about performing the proximity check on each significant location change, you could use a spatial indexing/search method like R-trees or R*-tree to reduce the number of comparisons needed for each location change, as those search algorithms will filter out (possibly large) spatially irrelevant regions. That should reduce the time/battery power needed to perform the proximity checks.

Answer 5

I know this post is old, but for those looking to do something similar, Skyhook offers the ability to geofence an infinite number of venues.

From their marketing: Skyhook’s Context Accelerator enables app developers and advertisers to instantly deploy Infinite Geofences to any brand chain (such as CVS) or venue category (such as convenience stores) through a simple web interface. Using the same patented technology from Skyhook’s first-party location network, the Context Accelerator SDK manages those active geofences on-device, regardless of OS limitations allowing for infinite geofencing.