Geofencing for CLBeacons - ios

Question

I am trying to establish a geofence for CLbeacons, which is like this :
a> Any beacon whose accuracy <= 2.5 metres of distance should get detected.
Now, when I place the beacons in about 7m distance apart both get detected. What is more shocking is that the accuracy sometimes goes like 15.70 m for the beacon (checked by running the Airlocate App), which happens randomly and thereby makes the geofencing thing impossible to construct.
I tried to apply the custom formula to calculate the beacon distance double accuracy = (0.89976) * pow(ratio,7.7095) + 0.111; where double ratio = rssi*1.0/txPower; but since the txPower for CLbeacons are not provided, the function depends on me providing a static value as txPower.
Can anyone guide as to how the geofencing for these CLBeacons should be constructed then?

Answer 1

You are correct in that the accuracy value of the beacon can fluctualte drastically over short periods. The way I handle this (we have a similar need to determining when devices have been returned to a base location) is a combination of two approaches: First, we are tweaking the power on our iBeacons to lower them so that the didDetermineState: delegate call does not get called to many times for entering and leaving the beacon's range. Second, my iBeacon model keeps track of the accuracy for any beacons in range and averages them out. That way someone walking in between the device and the beacon, or the user turning the device a particular way won't cause the huge fluctuations in the accuracy value, messing up your logic.
I don't believe Apple intended developers to use iBeacons as indoor geolocation. The geofencing aspect of it is to simply adjust the transmit power so that you can get notified of when your device can detect the signal or not. The accuracy can be used, but it is so inaccurate it should be used with caution.
There is a developer that claims to have developed an algorithm for using iBeacons for indoor positioning, but I have not experience with it. Also, if it were possible with any level of accuracy, I feel that Apple would be using it for it's indoor location capabilities, which they are not.
Here's some of the code I use:
Here's my custom MyBeacon class:
#interface MyBeacon()
#property NSMutableDictionary *accuracyHistory;
#end
#implementation MyBeacon
- (id) init
{
self = [super init];
if (self!=nil) {
self.accuracyHistory = [[NSMutableDictionary alloc] init];
}
return self;
}
- (void) addAccuracyValue:(CGFloat)rangeValue forDate:(NSDate *)rangeDateTime
{
[self removeOldRangeHistoryItems];
[self.accuracyHistory setObject:[NSNumber numberWithFloat:rangeValue] forKey:rangeDateTime];
}
- (double) getBeaconAverageAccuracy
{
[self removeOldRangeHistoryItems];
if( self.accuracyHistory.count == 0 )
{
return -1;
}
CGFloat sumRangeVals = 0.0;
int numRangeVals = 0;
for(NSDate *accuracyDateTime in self.accuracyHistory) {
NSNumber *curValue = [self.accuracyHistory objectForKey:accuracyDateTime];
if( [curValue floatValue] >= 0.0 )
{
sumRangeVals += [curValue floatValue];
numRangeVals++;
}
else // let's toy with giving unknown readings a value of 30.
{
sumRangeVals += 30;
numRangeVals++;
}
}
CGFloat averageRangeVal = sumRangeVals / numRangeVals;
return averageRangeVal;
}
- (void) removeOldRangeHistoryItems
{
NSMutableArray *keysToDelete = [[NSMutableArray alloc] init];
for(NSDate *accuracyDateTime in self.accuracyHistory) {
// remove anything older than 10 seconds.
if( [accuracyDateTime timeIntervalSinceNow] < -10.0 )
{
[keysToDelete addObject:accuracyDateTime];
}
}
for( NSDate *key in keysToDelete )
{
[self.accuracyHistory removeObjectForKey:key];
}
}
#end