[overengineer]

> The metre was re-defined[0] in 1791 as one ten-millionth of the quarter-meridian

Doesn't sound like a good idea. Circumference of earth around Paris is not unchangeable.

[fsloth]

Exact geographic measurements where the high technology of 18th century. Each society uses the highest precision measurements available to them. Also, plate tectonics were scifi fantasy until 1950's.

The definition has been updated after that. https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...

[delta_p_delta_x]

> Doesn't sound like a good idea

Which is why the CGPM changed the definition of the metre to:

- the length of the International Metre (a 9:1 platinum-iridium bar) in 1889;

- and then to 1650763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum in 1960;

- and finally to the modern definition, the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second in 1983.

This means the realisation of the metre depends on the definition of the second.