AccuLevel

AccuLevel is an app that uses augmented reality to measure angles, test the alignment of structures and know if something is horizontal or vertical with your camera. You can also see the angle of your device, and the surface where it's placed, on your Apple Watch, allowing the use of AccuLevel as a remote level.
You only need to align a reference line on your display with the image on the camera, and the tilt sensors will do the rest to get the angle.
The app is touch-customizable. You can zoom in and out of the displayed scene, change the reference line orientation from horizontal to vertical or change its position with one touch.
Caution: You must avoid parallax errors. They appear when the camera isn't properly aligned with the objects to measure. To minimize these errors, align the horizontal indicators on the border of the display with the horizon. Also place the camera on the same horizontal plane as the object with the help of the on screen device pitch indicator.