Getting an accurate location with CLLocationManager - iphone

Question

I'm beginning to mess around with the CLLocationManager and I am trying to use the callback method below. I went outside to test it on my iPhone, and with the first few updates, I seem to be getting gibberish values, like all of a sudden my distance is 39meters even though I haven't moved anywhere. Or sometimes it will start at 5, and then jump to 20 meters, again without me moving anywhere. I went outside and walked, and the updates from the initial starting point seemed 'ok,' and then when I walked back, I got back to the original 39 meters starting point. I was wondering if what I am doing below is correct. I also included my viewDidLoad method where I initialize my CLLocationManager object.
Is there a way to ensure that my first values are accurate? Thanks in advance.
- (void)viewDidLoad
{
[super viewDidLoad];
locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.distanceFilter = kCLDistanceFilterNone;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
}
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation
{
if (startingPoint == nil) {
NSLog(#"NIL starting point");
self.startingPoint = newLocation;
}
CLLocationDistance distance = [newLocation distanceFromLocation:startingPoint];
NSString *distanceString = [[NSString alloc] initWithFormat:#"%g m", distance];
distanceLabel.text = distanceString;
[distanceString release];
numberOfUpdates++;
NSString *countString = [[NSString alloc] initWithFormat:#"%d", numberOfUpdates];
countLabel.text = countString;
[countString release];
}

Answer 1

GPS is an imperfect technology. Atmospheric conditions, sattelite availability (and position), sky visibility, signals bouncing off nearby objects (buildings) all contribute to it's inherent inaccuracy. Though there is even an "accuracy" value (which is usually pretty good) - even this is not completely reliable.
Airplanes are not allowed to use GPS for precision approaches - even their receivers aren't accurate enough, and they require other technologies (which have their own issues).
Try running the standard "Maps" application and use it as a comparison. I think your code is good - it's jut GPS.
Of course I am saying this because I am working on my own maritime navigation application, an running into all these issues myself.

Answer 2

Though this is an old question I still like to answer. The first call to "didUpdateToLocation" usually is some old value and you should always check the timestamp. From the Apple documentation:
NSDate* eventDate = newLocation.timestamp;
NSTimeInterval howRecent = [eventDate timeIntervalSinceNow];
if (abs(howRecent) < 15.0)
{
// use the value
}

Answer 3

In your didUpdate method, you can test newLocation's .horizontalAccuracy property for an acceptable value and toss any values that are impossible (or even unlikely!).