Vertical Measurement can helps you to check anthing vertical or not.

Click Vertical Measure button to use this function. And you'll find a hint "Turn on the camera?".

If you choose NO, one emulational vertical appears and it's always perpendicular to the ground.

If you choose YES, the phone's camera turns on and the picture camera catched will be displayed into the phone screen. It can help you to observe angles intuitively.