[1]: https://en.wikipedia.org/wiki/Earth's_circumference
[2]: https://en.wikipedia.org/wiki/Metre#Meridional_definition
They spent several years making lots of smaller measurements that were added up.
Each measurement was done twice to ensure correctness. One of the distances had two conflicting measurements, but due to a war, they could not return to make a third measurement and had to just choose one of them (the wrong one).
They chose not to tell anyone because they feared politicians would use it to discredit the metric system.
>o their 1793 measurement of the distance from the equator to the North Pole was off by less than 2 km....