Doing some housework the other day I downloaded an app to my smartphone that included a level for measuring tilt. The app requires me to calibrate the level by first placing my phone on a perfectly straight surface. But why is this required?
I thought smartphones use MEMS sensors that measure the tilt angle with respect to gravity. This is what a traditional level does; it measures the angle so that when the surface is straight or at zero degrees it is perpendicular to the direction of gravity, and they do not need to be calibrated since the direction of gravity is the same always.