There is a great deal of information available on-line about physical standards and calibration techniques. Many clever people have been working on developing the techniques for hundreds of years. There is much to know.
Laser-based precision measurement techniques use optical interferometry. That allows distances to be measured down to a fraction of a wavelength of light. A wavelength of visible light is about 0.5 microns or 20 microinches.
Really accurate laser distance systems are calibrated for the speed of light in the local air since wavelengths go down as the speed goes down. If you know the frequency you know the wavelength and it is possible to measure the frequencies of laser light to about 15 places.
You have probably heard of "atomic clocks" that are very accurate. In fact the standard of time and frequency is a very precise resonance in cesium atoms and time and frequency are now defined in terms of that cesium resonance. The accuracy of such systems allows GPS among other things. A GPS receiver can know the correct time to within 20 billionths of a second because it is talking to multiple atomic clocks on multiple satellites.
For about 20 years (until the 1940s) a special pendulum clock was the standard of time measurement. Here is some info on that:
In 1984 Pierre Boucheron studied the accuracy of a Shortt clock preserved at the US Naval Observatory.[3][18] Using modern optical sensors which detected the precise time of passage of the pendulum without disturbing it, he compared its rate to an atomic clock for a month. He found that it was stable to 200 microseconds per day (2.31 ppb), equivalent to an error rate of one second in 12 years, far more accurate than the 1 second per year that was previously measured. His data revealed the clock was so sensitive it was detecting the slight changes in gravity due to tidal distortions in the solid Earth caused by the gravity of the Sun and Moon.[19]