I want to manually measure the dimension of a pattern printed on a flat surface, with an accuracy of about 0.1mm. For simplicity sake, let's say this is a black rectangle, printed on a white background, and i want to measure its width and height. The dimension can be between 20cm and 80cm.
I have tried with a vernier caliper, but placing the caliper accuratly enough on the rectangle borders is impossible, especially since the caliper jaws do not lie flat on the surface.
What kind of measuring instrument could i use? Is there something dedicated to this task, or a way to use a standard instrument to do that?