How do you measure the distance between two points on a curved surface and what instrument would you use?

There is an instrument called an opisometer which is like a wheel on the end of a stick. You can roll the wheel along the line on the curved surface and it counts the revolutions. Distance can be calculated from this. Alternatively, for measuring short distances you can roll it backwards along a straight line until the wheel is back in the start position and measure how far you've gone.