How do I point compass at my destination?

Question

I am creating an app that will point a simple arrow in the direction of a GPS coordinate I have plotted out. So I am talking my current location lat and long, so lat1, lon1 is the current location... then comparing them against a static coordinate of lat and long called lat2, lon2.

I have spent HOURS researching and testing, but I keep on getting the same problem. It's very aggravating. As soon as I start to drive towards my destination the arrow is all out of whacked when using the iPhone's built in GPS/compass. It will sometimes show the arrow the complete opposite side of where it needs to be, hanging to the right... pretty much nothing that is solid.

Here is the current code I am running

cpLocLat = 43.026629;

cpLocLon = -78.867188;

Those are float values and the coordinates of where I want to drive to.

float lat1 = self.locManager.location.coordinate.latitude;
float lat2 = cpLocLat;
float lon1 = self.locManager.location.coordinate.longitude;
float lon2 = cpLocLon;

float heading = atan2(sin(lon2 - lon1) * cos(lat2), cos(lat1) * sin(lat2) - sin(lat1) * cos(lat2) * cos(lon2 - lon1));

heading = RADIANS_TO_DEGREES(heading);

if (heading < 0) {
    heading = heading + 360;
}

I use this function every time the locationManager didUpdateToLocation delagate is called. Once I have that information, it is used in the locationManager didUpdateHeading delagte with the following:

CGAffineTransform where  = CGAffineTransformMakeRotation(degreesToRadians((degrees - newHeading.magneticHeading)));

[self.compassContainer2 setTransform:where];

and here is the degressToRadians macro

#define degreesToRadians(x) (M_PI * x / 180.0)

So whenever I run this code it is failing epically and is making me go nuts!

BTW - I have used the following URLs for reference

http://www.shawngrimes.me/2011/07/calculating-heading-with-corelocation/

Using Compass to point to actual coordinate location instead of just North

Point to location using compass

Answer 1

To my knowledge, the formula for the initial bearing according to the Haversine formula is:

θ = atan2( sin(Δlong).cos(lat2), cos(lat1).sin(lat2) − sin(lat1).cos(lat2).cos(Δlong) )

which seems to be exactly what you used. According to the man entry, you might have to use the reverse order of the two arguments to atan2:

ATAN2(3)                 BSD Library Functions Manual                 ATAN2(3)

NAME
     atan2 -- arc tangent function of two variables

SYNOPSIS
     ....
     float
     atan2f(float y, float x);

So y comes before x. Maybe that is the error.
Also, make sure that the coordinate deltas (lat2-lat1 etc.) are in radians.