Having trouble calculating accurate total walking/running distance using CLLocationManager

Question

I'm trying to build an iOS app that displays the total distance travelled when running or walking. I've read and re-read all the documentation I can find, but I'm having trouble coming up with something that gives me an accurate total distance.

When compared with Nike+ GPS or RunKeeper, my app consistently reports a shorter distance. They'll report the same at first, but as I keep moving, the values of my app vs other running apps gradually drift.

For example, if I walk .3 kilometers (verified by my car's odometer), Nike+ GPS and RunKeeper both report ~.3 kilometers every time, but my app will report ~.13 kilometers. newLocation.horizontalAccuracy is consistently 5.0 or 10.0.

Here's the code I'm using. Am I missing something obvious? Any thoughts on how I could improve this to get a more accurate reading?

#define kDistanceCalculationInterval 10 // the interval (seconds) at which we calculate the user's distance
#define kNumLocationHistoriesToKeep 5 // the number of locations to store in history so that we can look back at them and determine which is most accurate
#define kValidLocationHistoryDeltaInterval 3 // the maximum valid age in seconds of a location stored in the location history
#define kMinLocationsNeededToUpdateDistance 3 // the number of locations needed in history before we will even update the current distance
#define kRequiredHorizontalAccuracy 40.0f // the required accuracy in meters for a location.  anything above this number will be discarded

- (id)init {
    if ((self = [super init])) {
        if ([CLLocationManager locationServicesEnabled]) {
            self.locationManager = [[CLLocationManager alloc] init];
            self.locationManager.delegate = self;
            self.locationManager.desiredAccuracy = kCLLocationAccuracyBestForNavigation;
            self.locationManager.distanceFilter = 5; // specified in meters
        }

        self.locationHistory = [NSMutableArray arrayWithCapacity:kNumLocationHistoriesToKeep];
    }

    return self;
}

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
    // since the oldLocation might be from some previous use of core location, we need to make sure we're getting data from this run
    if (oldLocation == nil) return;
    BOOL isStaleLocation = [oldLocation.timestamp compare:self.startTimestamp] == NSOrderedAscending;

    [self.delegate locationManagerDebugText:[NSString stringWithFormat:@"accuracy: %.2f", newLocation.horizontalAccuracy]];

    if (!isStaleLocation && newLocation.horizontalAccuracy >= 0.0f && newLocation.horizontalAccuracy < kRequiredHorizontalAccuracy) {

        [self.locationHistory addObject:newLocation];
        if ([self.locationHistory count] > kNumLocationHistoriesToKeep) {
            [self.locationHistory removeObjectAtIndex:0];
        }

        BOOL canUpdateDistance = NO;
        if ([self.locationHistory count] >= kMinLocationsNeededToUpdateDistance) {
            canUpdateDistance = YES;
        }

        if ([NSDate timeIntervalSinceReferenceDate] - self.lastDistanceCalculation > kDistanceCalculationInterval) {
            self.lastDistanceCalculation = [NSDate timeIntervalSinceReferenceDate];

            CLLocation *lastLocation = (self.lastRecordedLocation != nil) ? self.lastRecordedLocation : oldLocation;

            CLLocation *bestLocation = nil;
            CGFloat bestAccuracy = kRequiredHorizontalAccuracy;
            for (CLLocation *location in self.locationHistory) {
                if ([NSDate timeIntervalSinceReferenceDate] - [location.timestamp timeIntervalSinceReferenceDate] <= kValidLocationHistoryDeltaInterval) {
                    if (location.horizontalAccuracy < bestAccuracy && location != lastLocation) {
                        bestAccuracy = location.horizontalAccuracy;
                        bestLocation = location;
                    }
                }
            }
            if (bestLocation == nil) bestLocation = newLocation;

            CLLocationDistance distance = [bestLocation distanceFromLocation:lastLocation];
            if (canUpdateDistance) self.totalDistance += distance;
            self.lastRecordedLocation = bestLocation;
        }
    }
}

Answer 1

As it turns out, the code I posted above works great. The problem happened to be in a different part of my app. I was accidentally converting the distance from meters to miles, instead of from meters to kilometers. Oops!

Anyway, hopefully my post will still have some merit, since I feel it's a pretty solid example of how to track a user's distance with Core Location.