Question

I’m trying to write a category for CLLocation to return the bearing to another CLLocation.

I believe I’m doing something wrong with the formula (calculous is not my strong suit). The returned bearing is always off.

I’ve been looking at this question and tried applying the changes that were accepted as a correct answer and the webpage it references:

Calculating bearing between two CLLocationCoordinate2Ds

http://www.movable-type.co.uk/scripts/latlong.html

Thanks for any pointers. I’ve tried incorporating the feedback from that other question and I’m still just not getting something.

Thanks

Here’s my category –

—– CLLocation+Bearing.h

#import <Foundation/Foundation.h>
#import <CoreLocation/CoreLocation.h>


@interface CLLocation (Bearing)

-(double) bearingToLocation:(CLLocation *) destinationLocation;
-(NSString *) compassOrdinalToLocation:(CLLocation *) nwEndPoint;

@end

———CLLocation+Bearing.m

#import "CLLocation+Bearing.h"

double DegreesToRadians(double degrees) {return degrees * M_PI / 180;};
double RadiansToDegrees(double radians) {return radians * 180/M_PI;};


@implementation CLLocation (Bearing)

-(double) bearingToLocation:(CLLocation *) destinationLocation {

 double lat1 = DegreesToRadians(self.coordinate.latitude);
 double lon1 = DegreesToRadians(self.coordinate.longitude);

 double lat2 = DegreesToRadians(destinationLocation.coordinate.latitude);
 double lon2 = DegreesToRadians(destinationLocation.coordinate.longitude);

 double dLon = lon2 - lon1;

 double y = sin(dLon) * cos(lat2);
 double x = cos(lat1) * sin(lat2) - sin(lat1) * cos(lat2) * cos(dLon);
 double radiansBearing = atan2(y, x);

 return RadiansToDegrees(radiansBearing);
}

Answer 1

Your code seems fine to me. Nothing wrong with the calculous. You don’t specify how far off your results are, but you might try tweaking your radian/degrees converters to this:

double DegreesToRadians(double degrees) {return degrees * M_PI / 180.0;};
double RadiansToDegrees(double radians) {return radians * 180.0/M_PI;};

If you are getting negative bearings, add 2*M_PI to the final result in radiansBearing (or 360 if you do it after converting to degrees). atan2 returns the result in the range -M_PI to M_PI (-180 to 180 degrees), so you might want to convert it to compass bearings, using something like the following code

if(radiansBearing < 0.0)
    radiansBearing += 2*M_PI;