Get angle from north using iOS api - objective-c

Question

Here is what i want:
http://postimage.org/image/9pq8m79hx/
I know the coordonates of the O point and of the X point. Is there any posibility to find the V angle (the angle from North orientation) using iOS methods?

Answer 1

Yep.
#import <math.h>
float a = -1 * atan2(y1 - y0, x1 - x0);
if (a >= 0) {
a += M_PI / 2;
} else if (a < 0 && a >= -M_PI / 2) {
a += M_PI / 2;
} else {
a += 2 * M_PI + M_PI / 2;
}
if (a > 2 * M_PI) a -= 2 * M_PI;
Now a will contain the angle in radians, in the interval 0...2 PI.
Doesn't even need any iOS-specific APIs. Remember: iOS still has all the features of libc.

Answer 2

not sure user529758's answer addresses the question, which I read as being about changing East as 0 degrees to North as 0 degrees. Code below works - key line is 4th line which changes 0 degrees from East to North
-(CGFloat) bearingFromNorthBetweenStartPoint: (CGPoint)startPoint andEndPoint:(CGPoint) endPoint {
// get origin point of the Vector
CGPoint origin = CGPointMake(endPoint.x - startPoint.x, endPoint.y - startPoint.y);
// get bearing in radians
CGFloat bearingInRadians = atan2f(origin.y, origin.x);
// convert to bearing in radians to degrees
CGFloat bearingInDegrees = bearingInRadians * (180.0 / M_PI);
// convert the bearing so that it takes from North as 0 / 360 degrees, rather than from East as 0 degrees
bearingInDegrees = 90 + bearingInDegrees;
// debug comments:
if (bearingInDegrees >= 0)
{
NSLog(#"Bearing >=0 in Degrees %.1f degrees", bearingInDegrees );
}
else
{
bearingInDegrees = 360 + bearingInDegrees;
NSLog(#"Bearing in Degrees %.1f degrees", bearingInDegrees );
}
return bearingInDegrees;
}