The meridian in the church of Saint-Sulpice serves to measure the solar calendar hour of noon. The meridian at the Museum of Louvre commemorates the meridian zero used in cartography until 1884. The two are parallel and located about 300 meters apart.
In 1792 Louis XVI makes his last executive decision to calculate off the length of the meridian the universal measurement which they call meter. In 2003 Robert Langdon, the DaVinci Code protagonist, follows the Rose Line from Louvre to Saint-Sulpice. How the mathematicians arrived at meter is pretty simple, meter is a ten millionth of a quarter of the meridian. As to how Robert Langdon following one line gets to the mark on the other, I have no idea.