Question

My first location using Core Location is almost always invalid.

My code is as follows:

- (void)locationManager:(CLLocationManager *)manager
    didUpdateToLocation:(CLLocation *)newLocation
           fromLocation:(CLLocation *)oldLocation
{ 
    //for saving the data ACCURATELY for calculations
    // make sure the coordinates are valid
    if ([self isValidLocation:newLocation withOldLocation:oldLocation])
    {         
        mDistance = [newLocation distanceFromLocation:oldLocation];

        [[NSNotificationCenter defaultCenter]postNotificationName:@"locationUpdated" object:nil];
    }
}

As you can see it checks to see if it is a valid location with isValidLocation. That code is here:

- (BOOL)isValidLocation:(CLLocation *)newLocation
        withOldLocation:(CLLocation *)oldLocation
{    
    // Throw away first point
    if (isFirstPoint)
    {
        NSLog(@"First point thrown away.");
        isFirstPoint = NO; //subsequent updates will NOT be the first point
        return NO;
    }
    // Filter out nil locations
    if (!newLocation){
        NSLog(@"New location invalid");
        return NO;
    }
    if (!oldLocation)
    {
        NSLog(@"Old location invalid");
        return NO;
    }
    // Filter out points by invalid accuracy
    if (newLocation.horizontalAccuracy < 0)
    {
        return NO;
    }
    // Filter out points that are out of order
    NSTimeInterval secondsSinceLastPoint = [newLocation.timestamp 
                                            timeIntervalSinceDate:oldLocation.timestamp];
    if (secondsSinceLastPoint < 0){
        return NO;
    }
    // Filter out points created before the manager was initialized
    NSTimeInterval secondsSinceManagerStarted = [newLocation.timestamp 
                                                    timeIntervalSinceDate:locationManagerStartDate];
    if (secondsSinceManagerStarted < 0){
        return NO;
    }
    // If the distance is negative
    if ([newLocation distanceFromLocation:oldLocation] < 0)
    {
        return NO;
    }
    if(newLocation.speed < 0) 
    {
        return NO;
    }
    // GIANT ELSE: The newLocation is good to use
    return YES;
} 

Even after checking all of that, my first point is always invalid. For example, I went from work to home, and turned on my location manager at home, yet my first coordinate was from my office at work. How can I fix this?

Here is what happens when I get an invalid first point:

Answer 1

It really depends on your use case. Core Location often returns the device’s last known position in order to give you a result as quickly as possible. This location is passed to you before the device has finished acquiring a new location. The last known position can be several kilometers off your current position.

Depending on the type of your app, this might be a totally negligible difference (e.g. if an app just wants to locate the country a user is in) or it might represent an inacceptable error. Core Location can’t tell.

It’s important for you to decide how to deal with this behavior. Always throwing the first reported location away might not be the best strategy. You might want to store the time when you call startUpdatingLocation and only throw the first reported location away if the system passes it to you within a very brief period of time (say, 1/10th of a second) that makes it likely to be a cached location. Also, the location’s timestamp property might be helpful in judging how (in)accurate the reported location can be.