iPhone - Which direction is positive theta?

Question

I wanted to use the iPhone's rotation system to be able to have an object follow below a line the user rotates. To do this, I need the angle at which the line is being measured as. When I set the rotation equal to M_PI_2, the object rotates 90 degrees counterclockwise. Which begs the question: Is the iPhone's angle-measurement system backwards? In other words, in portrait orientation, screen facing you, is positive theta clockwise?

Thanks.

Answer 1

I figured it out after doing some researching. Since the iPhone's +x axis is to the right (same orientation as question) and the +y axis is down, it should make sense that the +theta direction is clockwise.

The normal axes

The iPhone's Axes