This is a millennium project launched in the year 2000 by the government, commemorating one of the last acts of Louis XVI before the revolution, and his beheading.
In the midst of preparations for his escape, he gave his royal blessing to the survey of the meridian of the Paris Observatory, from Dunkirk to Barcelona.
This would fix the circumference of the earth and lead to the new immutable unit of length, the metre, defined as one ten-millionth of the distance from the north pole to the equator (10,000 km).
It took seven years and the geometers were set upon repeatedly by hostile peasants speaking unrecognisable languages.
The new project aims to plant a continuous band of trees along this line.