The science of defining measurements is a strange pursuit, and very much a quest to pin down those things which refuse to be immutably static. We have come exceedingly close to exact on a number of the important basics, like time and distance. We do that with methods and materials that can be recreated in laboratories anywhere on Earth, just the same as any published research should be replicable by a group of scientific peers. We have the second bound to the predictable radiation patterns of a caesium atom. The meter is a fixed interval of laser light traveling in a vacuum.