Use segments to determine angles
If this option is selected in the Preferences window the angle tool will change its appearance to .
Accordingly, the angle tool will work differently, instead clicking three times to draw an angle, the user should select consecutively two segments previously drawn on the image.
After two segment are selected an angle based on the two selected segments will be automatically drawn.
If the proper intersection point for two lines (the angle vertex) is not calculated correctly, the resulting angle measurement will be wrong.
To fix that kind of error, right-click on the image and select Change Last Angle from Segments from the pop-up menu.
To invert the angle (from clockwise to counter-clockwise or vice versa), open the Measurements Pane and select "Inv. Angle" in the Options column.