Comment by lumost

At a large scale it is also very difficult to create high accuracy. Simply defining a meter or kilogram is a high difficulty task, however there is a canonical "kilogram" that you can go and visit in France. Along with a carefully maintained set of proofs that were built off of the canonical "kilogram" which in turn are used to make the calibration weights we all work with. We then measure/calibrate the accuracy of any weight measuring system by how well it tracks to the canonical kilogram.

Similarly, for any "new" measurement, there will be a number of "calibration" measurements performed to ensure that the results are in-line with other measurements of well known things.