objective c - Corelocation incorrect distances

Question

I am developing an application which computes the distance traveled by the user. I am using CLLocationManager class to do so, but I am getting the cached data initially and also the distance variable is increasing at a sudden rate. Please help me out... I have used the following code....

Note:distance is a static var. here

  - (void)viewDidLoad {
    [super viewDidLoad];
//bestEffortAtLocation = nil;
oldLocat = [[CLLocation alloc]init];
newLocat = [[CLLocation alloc]init];
locationManager =[[CLLocationManager alloc]init];
locationManager.delegate = self;
locationManager.distanceFilter =  kCLDistanceFilterNone;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
[locationManager startUpdatingLocation];

  }

- (void)locationManager:(CLLocationManager *)manager
didUpdateToLocation:(CLLocation *)newLocation
       fromLocation:(CLLocation *)oldLocation{




// test that the horizontal accuracy does not indicate an invalid measurement
if (newLocation.horizontalAccuracy < 0) return;


NSLog(@"accuracy %d",newLocation.horizontalAccuracy);

// test the age of the location measurement to determine if the measurement is cached
// in most cases you will not want to rely on cached measurements
NSTimeInterval locationAge = -[newLocation.timestamp timeIntervalSinceNow];
//NSLog(@"time %d",locationAge);

if (locationAge > 5.0) return;



self.oldLocat = oldLocation;
self.newLocat = newLocation;


double latDegrees = newLocation.coordinate.latitude;
NSString *lat = [NSString stringWithFormat:@"%1.5f°",latDegrees];
latLabel.text = lat;
double longDegrees = newLocation.coordinate.longitude;
NSString *longt = [NSString stringWithFormat:@"%1.5f°",longDegrees];
longLabel.text = longt;
[self computeDistanceFrom:oldLocat tO:newLocat];

   }

    -(void)computeDistanceFrom:(CLLocation *)oldL tO:(CLLocation *)newL
 {
NSLog(@"oldd %@",oldL);
NSLog(@"new %@",newL);


distance = distance + [oldL getDistanceFrom:newL];
NSLog(@"distance %f",distance);

}

The console is displaying the following data.......

accuracy 0 oldd (null) new <+28.62114850, +77.37001021> +/- 80.00m (speed -1.00 mps / course -1.00) @ 2010-06-22 19:21:59 +0530 distance 0.000000

accuracy 0 oldd <+28.62114850, +77.37001021> +/- 80.00m (speed -1.00 mps / course -1.00) @ 2010-06-22 19:21:59 +0530 new <+28.61670485, +77.37068155> +/- 80.00m (speed -1.00 mps / course -1.00) @ 2010-06-22 19:22:00 +0530 distance 498.211345

accuracy 0 oldd <+28.61670485, +77.37068155> +/- 80.00m (speed -1.00 mps / course -1.00) @ 2010-06-22 19:22:00 +0530 new <+28.62112748, +77.36998540> +/- 80.00m (speed -1.00 mps / course -1.00) @ 2010-06-22 19:23:02 +0530 distance 994.432508

Answer 1

It is normal to initially get a cached location from before. You can ignore older cached data by looking at the timestamp of the CLLocation.

You are printing the accuracy incorrectly, use %f not %d, type is double not int.

Location can change quickly when GPS first starts because you have a low accuracy location from cell triangulation, then as you get GPS acquisition you get a higher accuracy location. Those can be far apart (1000m) and it appears that you moved far in a few seconds but only the accuracy has changed.

Don't use two locations that have very different accuracy for computing distance traveled.

EDIT Added code sample, how to ignore old location data. You decide how old to ignore, I used 60 seconds here:

- (void)locationManager:(CLLocationManager *)manager 
    didUpdateToLocation:(CLLocation *)newLocation 
           fromLocation:(CLLocation *)oldLocation {

    NSTimeInterval ageInSeconds = -[newLocation.timestamp timeIntervalSinceNow];
    if (ageInSeconds > 60.0) return;   // data is too long ago, don't use it

    // process newLocation
    ...
}