Angle to Mecca from current location with iPhone compass

Question

I am developing an iPhone application that will display both the North and Qibla directions. I am displaying the North direction with the aid of CLLocationManager and updating it with CLHeading as newHeading. magneticHeading, And using the following code, I am indicating the Qibla direction.

double A = MECCA_LONGITUDE - lon;
double b = 90.0 - lat;
double c = 90.0 - MECCA_LATITUDE;
NSLog(@"tan -1( sin(%f) / ( sin(%f) * cot(%f) - cos(%f) * cos(%f)))", A, b, c, b, A);
double qibAngle =  atan(sin(A) /( sin(b) * (1 / tan(c)) - cos(b) * cos(A) ));
NSLog(@"qib Angle- %f",qibAngle);
qibla.transform = CGAffineTransformMakeRotation(qibAngle * M_PI /180);

I'm getting the angle here, but when I rotate the device, the angle does not change. Could someone please help me? I know that I need to do something with the title, but I'm not sure what.

Answer 1

The code you posted, I think, calculates the angle between the location's geographic north and the direction toward Mecca. Now all you have to do is consider the user's heading.

Consider a situation where the user is facing directly due East and Mecca is directly due West. The qibla angle must be -90 degrees because tan returns +/-90 degrees. The adjustment should now be clear: to find the angle that the user must turn to face Mecca, you must subtract 90 degrees from the qibla's angle with regard to geographical north (-90).

In other words, you need to "undo" the user's deviance, and you accomplish this by

the user's heading, as it relates to the geographic north, is known as the qibla angle.

Now that the math is out of the way, you must watch for heading changes and recalculate the qibla angle as necessary. Remember to use the trueHeading attribute, too.