Well UIScrollView's already have a built in pan gesture that you could tap into. Usage would be as simple as setting your class as your scroll view's delegate (to utilize scrollViewWillBeginDragging) and using UIPanGestureRecognizer's -locationInView: to determine touch location.
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
CGPoint location = [scrollView.panGestureRecognizer locationInView:scrollView];
NSLog(@"%@",NSStringFromCGPoint(location));
}