Why does measurement need an epistemology and what could it look like?

Measuring things seems quite straightforward. Most of us already learned in primary school to measure how far apart two points are on a piece of paper: draw a line between them, and compare its length with the length of your ruler. After checking how many marks on the ruler your line covers, you can express the outcome of your measurement numerically. You may now see that the points are 10 centimeters or 4 inches apart, but the knowledge of such units is hardly necessary for your purposes....