Clocks, more precisely optical atomic clocks, can be used for geodetic observations. This could become a very important operational technique in the future.
When frequencies generated by clocks at different sites are compared at this level of precision (10-18), the comparison contains the relativistic gravitational frequency redshift due to the different height (and gravity potential) of the sites, as described by General Relativity.
This can be used to determine height differences by converting frequency differences. The results of chronometric leveling are comparable to the determination of height and potential differences by classical methods such as first order spirit leveling or the gravimetric GNSS/geoid approach. In 2019, in a very promising experiment using the world’s best transportable optical atomic clocks, chronometric leveling has been demonstrated to an accuracy of 4 cm over a height difference of 450 m.
Measure Heights with Clocks