senile

Hello

I used a digital angle gauge recently, how does it work?
Is there a (MEMS) gyroscope inside perhaps?
But isn't it that gyroscopes need a reference when turned on, and this one showed 180 deg reading when turned on upside down.

My digital camera senses whether it is in a vertical or horizontal position and automatically rotates the photos respectively. Do you have ideas about the technology behind this?
What kind of sensors there are to measure the angle respect to gravity?