not getting the exact speed when driving a car - iphone

Question

i`m building an app wich notify the user when getting to the max speed limit "120 km/h"
but i`m not getting the exact speed value when testing the app while driving
lets say i`m driving at 80 km/h speed , the value i get from the app is a range between 60 - 100 sometimes more changing every second.
this is my whole code i`m using
viewController.h
#import <UIKit/UIKit.h>
#import <CoreLocation/CoreLocation.h>
#import "CoreLocationController.h"
#import <MapKit/MapKit.h>
#interface ViewController : UIViewController <CLLocationManagerDelegate, CoreLocationControllerDelegate> {
MKMapView *mapview;
CLLocationManager *locationManager;
IBOutlet UILabel *speedLabel;
CoreLocationController *CLController;
int distance;
int currentSpeed;
int maxSpeed;
IBOutlet UILabel *distanceLbl;
IBOutlet UILabel *maxSpeedLbl;
IBOutlet UILabel *currentSpeedLbl;
}
- (IBAction)Getlocation:(id)sender;
#property (nonatomic, retain) IBOutlet UILabel *distanceLbl;
#property (nonatomic, retain) IBOutlet UILabel *maxSpeedLbl;
#property (nonatomic, retain) IBOutlet UILabel *currentSpeedLbl;
#property (retain, nonatomic) IBOutlet UILabel *speedLabel;
#property (nonatomic, retain) CoreLocationController *CLController;
#property (retain, nonatomic) IBOutlet MKMapView *mapview;
#property (strong, nonatomic) IBOutlet CLLocationManager *locationManager;
viewController.m
#synthesize distanceLbl,maxSpeedLbl,currentSpeedLbl;
#synthesize locationManager;
#synthesize mapview;
#synthesize speedLabel;
#synthesize CLController;
- (void)locationChange:(CLLocation *)newLocation:(CLLocation *)oldLocation {
NSTimeInterval difference = [[newLocation timestamp] timeIntervalSinceDate:[oldLocation timestamp]];
double temp_distance = [newLocation getDistanceFrom:oldLocation];
distance += temp_distance;
distanceLbl.text = [NSString stringWithFormat:#"%.2d",distance];
currentSpeed = (temp_distance/difference) * (18.0/5.0);
if (currentSpeed > maxSpeed) {
maxSpeed = currentSpeed;
maxSpeedLbl.text = [NSString stringWithFormat:#"%.2d",maxSpeed];
}
currentSpeedLbl.text = [NSString stringWithFormat:#"%.2d",currentSpeed];
}
- (void)locationUpdate:(CLLocation *)location {
speedLabel.text = [NSString stringWithFormat:#"SPEED: %f", [location speed]];
self.mapview.showsUserLocation = YES;
}
- (void)locationError:(NSError *)error {
speedLabel.text = [error description];
}
- (IBAction)Getlocation:(id)sender {
locationManager = [[CLLocationManager alloc] init];
locationManager.distanceFilter = kCLDistanceFilterNone;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
locationManager.delegate = self;
[locationManager startUpdatingLocation];
[mapview setMapType:MKMapTypeStandard];
[mapview setZoomEnabled:YES];
[mapview setScrollEnabled:YES];
self.mapview.showsUserLocation = YES;
MKCoordinateRegion region = { {0.0, 0.0 }, {0.0, 0.0 } };
region.center.latitude = locationManager.location.coordinate.latitude;
region.center.longitude = locationManager.location.coordinate.longitude;
region.span.longitudeDelta = 0.007f;
region.span.latitudeDelta = 0.007f;
[mapview setRegion:region animated:YES];
[mapview setDelegate:sender];
}
- (IBAction)setMap:(id)sender {
switch (((UISegmentedControl *) sender).selectedSegmentIndex) {
case 0:
mapview.mapType = MKMapTypeStandard;
break;
case 1:
mapview.mapType = MKMapTypeSatellite;
break;
case 2:
mapview.mapType = MKMapTypeHybrid;
break;
default:
break;
}
}
- (void)viewDidLoad
{
[super viewDidLoad];
CLController = [[CoreLocationController alloc] init];
CLController.delegate = self;
[CLController.locMgr startUpdatingLocation];
[self Getlocation:self];
maxSpeed = 120;
}
CoreLocationController.h
#protocol CoreLocationControllerDelegate
#required
- (void)locationUpdate:(CLLocation *)location;
- (void)locationError:(NSError *)error;
- (void)locationChange:(CLLocation *)newLocation:(CLLocation *)oldLocation;
#end
#interface CoreLocationController : NSObject <CLLocationManagerDelegate> {
CLLocationManager *locMgr;
id delegate;
//new speed method
int distance;
int currentSpeed;
int maxSpeed;
IBOutlet UILabel *distanceLbl;
IBOutlet UILabel *maxSpeedLbl;
IBOutlet UILabel *currentSpeedLbl;
}
#property (nonatomic, retain) IBOutlet UILabel *distanceLbl;
#property (nonatomic, retain) IBOutlet UILabel *maxSpeedLbl;
#property (nonatomic, retain) IBOutlet UILabel *currentSpeedLbl;
#property (nonatomic, retain) CLLocationManager *locMgr;
#property (nonatomic, assign) id delegate;
CoreLocationController.m
#synthesize locMgr;
#synthesize delegate = _delegate;
#synthesize distanceLbl,maxSpeedLbl,currentSpeedLbl;
- (id)init {
self = [super init];
if(self != nil) {
self.locMgr = [[CLLocationManager alloc] init];
self.locMgr.delegate = self;
}
return self;
}
- (void)locationChange:(CLLocation *)newLocation:(CLLocation *)oldLocation {
NSTimeInterval difference = [[newLocation timestamp] timeIntervalSinceDate:[oldLocation timestamp]];
double temp_distance = [newLocation getDistanceFrom:oldLocation];
distance += temp_distance;
distanceLbl.text = [NSString stringWithFormat:#"%.2d",distance];
currentSpeed = (temp_distance/difference) * (18.0/5.0);
if (currentSpeed > maxSpeed) {
maxSpeed = currentSpeed;
maxSpeedLbl.text = [NSString stringWithFormat:#"%.2d",maxSpeed];
}
currentSpeedLbl.text = [NSString stringWithFormat:#"%.2d",currentSpeed];
}
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
if([self.delegate conformsToProtocol:#protocol(CoreLocationControllerDelegate)]) {
[self.delegate locationUpdate:newLocation];
[self.delegate locationChange:newLocation :oldLocation];
}
}
- (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error {
if([self.delegate conformsToProtocol:#protocol(CoreLocationControllerDelegate)]) {
[self.delegate locationError:error];
}
}