iPhone Gps logging inaccurate - iphone

Question

I'm logging gps points during a walk. Below it shows the function that the coordinates are saved each 5 seconds.
i Did several tests but i cannot get the right accuracy i want. (When testing the sky is clear also tests in google maps shows me that the gps signal is good).
here is the code:
-(void)viewDidAppear:(BOOL)animated{
if (self.locationManager == nil){
self.locationManager = [[[CLLocationManager alloc] init] autorelease];
locationManager.delegate = self;
// only notify under 100 m accuracy
locationManager.distanceFilter = 100.0f;
locationManager.desiredAccuracy= kCLLocationAccuracyBest;
[locationManager startUpdatingLocation];
}
}
- start logging
[NSTimer scheduledTimerWithTimeInterval:5 target:self selector:#selector(getData) userInfo:nil repeats:YES];
</code>
<code>
-(void)getData{
int distance;
// re-use location.
if ([ [NSString stringWithFormat:#"%1.2f",previousLat] isEqualToString:#"0.00"]){
// if previous location is not available, do nothing
distance = 0;
}else{
CLLocation *loc1 = [[CLLocation alloc] initWithLatitude:previousLat longitude:previousLong];
CLLocation *loc2 = [[CLLocation alloc] initWithLatitude:latGlobal longitude:longGlobal];
distance = [loc1 getDistanceFrom: loc2];
}
// overwrite latGlobal with new variable
previousLat = latGlobal;
previousLong = longGlobal;
// store location and save data to database
// this part goes ok
}
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
// track the time to get a new gps result (for gps indicator orb)
lastPointTimestamp = [newLocation.timestamp copy];
// test that the horizontal accuracy does not indicate an invalid measurement
if (newLocation.horizontalAccuracy < 0) return;
// test the age of the location measurement to determine if the measurement is cached
// don't rely on cached measurements
NSTimeInterval locationAge = -[newLocation.timestamp timeIntervalSinceNow];
if (locationAge > 5.0) return;
latGlobal = fabs(newLocation.coordinate.latitude);
longGlobal= fabs(newLocation.coordinate.longitude);
}
I have taken a screenshot of the plot results (the walk takes 30 minutes) and an example of what i'am trying to acomplish:
http://www.flickr.com/photos/21258341#N07/4623969014/
i really hope someone can put me in the right direction.

Answer 1

Looking at your plots - This is exactly what I see too in an app I am working on.
Looking at your code. Whenever location is updated, locationManager calls didUpdateToLocationFromLocation - polling at 5s intervals will give you a lot of points at the same location. (but not relevant to the question)
Also I think you are leaking a lastPointTimestamp (not relevant to the question)
Right - the jumping around - you can look at the accuracy of the points : you do this in locationManager anyway but only check for <0. Is there a pattern of what the horizontal accuracy is on the jumping points ? The accuracy figure may be much worse then for other points, or one specific value (caused by a switch to cell tower triangulation).
Finally, you may need to implement filtering. Kalman filters are the over-the-top way to go, but I am looking at using low-pass filters combined with some basic physics (max acceleration & speed for walking, cycling, running, driving etc) to improve results.
Happy to collaborate on this.