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我能够检索两点的度数并以该度数旋转图像。但为了显示方向，有时我需要在实际度数上加上 180，或者有时我不必加上度数。这是因为我正在使用自定义注释图像。任何形式的帮助将不胜感激。

- (double) getDirection
{

    //Determine the direction
    double lat1 = self.startCoOrdinate.latitude * M_PI / 180.0;
    double lon1 = self.startCoOrdinate.longitude * M_PI / 180.0;
    double lat2 = self.endCoOrdinate.latitude * M_PI / 180.0;
    double lon2 = self.endCoOrdinate.longitude * M_PI / 180.0;

    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);


    NSLog(@"degre between points is %f",(radiansBearing * 180.0) / M_PI);


    double degree = radiansBearing * 180.0 / M_PI;


    return (degree + 0);
}

- (UIImage*)rotatedImage:(UIImage*)sourceImage byDegreesFromNorth:(double)degrees {
    CGSize rotateSize =  sourceImage.size;
    UIGraphicsBeginImageContext(rotateSize);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(context, rotateSize.width/2, rotateSize.height/2);
    CGContextRotateCTM(context, ( degrees * M_PI/180.0 ) );
    CGContextScaleCTM(context, 1.0, -1.0);
    CGContextDrawImage(UIGraphicsGetCurrentContext(),
                       CGRectMake(-rotateSize.width/2,-rotateSize.height/2,rotateSize.width, rotateSize.height),
                       sourceImage.CGImage);
    UIImage *rotatedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return rotatedImage;
}
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