Measuring velocity via iPhone SDK

Question

I need to implement a native iPhone app to measure the velocity of the phone (basically a speedometer). I know that you can do so via the CoreLocation API fairly easily, bu

Answer 1

From a practical standpoint, you are not going get accurate velocity from forces acting on the accelerometer.

Use the GPS with readings taken at 1 minute intervals and put the GPS to sleep inbetween.

Here is an example:

SpeedViewController.h

CLLocationManager *locManager;
CLLocationSpeed speed;
NSTimer *timer;

@property (nonantomic,retain) NSTimer *timer;

SpeedViewController.m

#define kRequiredAccuracy 500.0 //meters
#define kMaxAge 10.0 //seconds

- (void)startReadingLocation {
    [locManager startUpdatingLocation];
}

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

    NSTimeInterval ageInSeconds = [newLocation.timestamp timeIntervalSinceNow];

    //ensure you have an accurate and non-cached reading
    if( newLocation.horizontalAccuracy > kRequiredAccuracy || fabs(ageInSeconds) > kMaxAge )
        return;

    //get current speed    
    currentSpeed = newLocation.speed;

    //this puts the GPS to sleep, saving power
    [locManager stopUpdatingLocation]; 

    //timer fires after 60 seconds, then stops
    self.timer = [NSTimer scheduledTimerWithTimeInterval:60.0 target:self selector:@selector(timeIntervalEnded:) userInfo:nil repeats:NO];
}

//this is a wrapper method to fit the required selector signature
- (void)timeIntervalEnded:(NSTimer*)timer {
    [self startReadingLocation];
}