Having trouble calculating accurate total walking/running distance using CLLocationManager - objective-c

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.

Answer 2

You probably have set kRequiredHorizontalAccuracy too low. If there is no location in the history that has accuracy < kRequiredHorizontalAccuracy, then you ignore all those points and add 0 to the distance.