Méridien de Paris
The metre adopted in 1795 by the French National Convention corresponds to the ten millionth part of the quarter of the terrestrial meridian. The Paris meridian crosses France from Dunkirk to Perpignan, passing through the Paris observatory and ... Yèvre-le-Châtel.
Under the Ancien Régime, units of measurement in France varied from town to town and province to province.
In order to establish a universal standard, "dedicated to all men and all times", the metre was officially defined in 1791 by the Académie des Sciences as the ten-millionth part of a quarter of a terrestrial meridian.
Astronomers Delambre and Mechain were commissioned to determine the meridian from Dunkirk to Barcelona by triangulation using sights on high points (towers, steeples, hills).
It took them almost seven years to complete their mission.
The definition of the metre, and simultaneously of the units of volume and mass, was definitively adopted by the French National Convention in 1795, creating the decimal metric system.