A spirit level or bubble level is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb).
Vials on modern level meter have a slightly curved glass tube which is incompletely filled with a liquid, usually a colored spirit or alcohol, leaving a bubble in the tube. At slight inclinations the bubble travels away from the center position, which is usually marked. Bubble Level app tries to mimic the real level meter and displays the data as real level meter would.
An extension of the spirit level is the bull's eye level: a circular, flat-bottomed device with the liquid under a slightly convex glass face with a circle at the center. It serves to level a surface across a plane, while the tubular level only does so in the direction of the tube. Bubble Level app tries to mimic the real bull's eye level and displays the data as real bull's eye level would.
Where any why use Bubble Level?
A bubble level is a device used in construction, carpentry and photography that determines if objects on which you're working are level. Used properly, a bubble level can help you create flawlessly level pieces of furniture, staircases and homes. A bubble level is also helpful when setting up a tripod for photographs or when hanging paintings or other items on the wall.
Bubble Level – Spirit Level