To unify the different units whose diversity complicated commerce, the Convention decided to create the metric system. Since this system was supposed to be universal, the meter had to be based on an unchangeable length that was the same for all nations: the length of the terrestrial meridian. Since the existing measurements of parts of the meridian were considered inadequate, the Convention sent two astronomers, Jean-Baptiste Delambre (1749-1822) and Pierre-François-André Méchain (1744-1804), to again measure the Paris meridian from Dunkirk to Barcelona. They succeeded in spite of all kinds of problems.

caption : Metric sphere - credits : BNF

To know more : The meter, a republican measurement