The Cassini Map of France: An Encyclopedia of the Territory

In 2021 and 2025, the National Institute of Geographic and Forest Information (IGN) transferred to the National Archives the collection of archives relating to the Academy's map of France, better known as the Cassini map of France.
This remarkable collection consists of calculations, writings, minutes, as well as original copper plates and prints recounting the manufacture and life of this map between 1750 and the 1950s. A collection whose exceptional nature allows us today to unveil unknown aspects of a work with universal reach; a work conceived by four generations of astronomers and cartographers: the Cassinis.

The art of science

The first map surveyed "geometrically" at the scale of an entire country, the kingdom of France, then the most powerful and populous in Europe, its realization is admirable in more ways than one. It is, first of all, the fruit of a continuous state will which, with Louis XIV and Colbert, Louis XV and then Bonaparte, decided on and perpetuated the project; even if, from 1756 to 1793, a private company financed it in part. It involves the use of innovative scientific techniques, for the method and instruments, which link it to the creation of the Academy of Sciences in 1666 and the Paris Observatory in 1667. It was born from the constancy of scholars who implemented it until 1815, alongside the dynasty of the four Cassinis from 1669 to 1793. It resulted from field work and then restitution by engraving, which was both meticulous, standardized, sometimes difficult, and very long-term, from the surveys of Abbé Picard in 1668 to the last surveys verified in 1790. Finally, it is extraordinary in its final deployment, in space of course since the 181 assembled sheets are more than twelve meters high and eleven meters wide; but also in time, because from the first works in 1668 to the final use of the copper plates in the 20th century, almost three centuries elapsed!

An extraordinary adventure

The construction of the Cassini map of France constitutes a true epic. From astronomer to bookseller, including engineer-cartographer, surveyor or engraver, each profession had to work with great precision and in compliance with the instructions detailed by Cassini III in 1744 in his Geometrical Description of France. Surveys, toponymy, calculations, drawings, signs… all steps are scrupulously respected. Objective lenses, reflecting circles, quadrants…, state-of-the-art instruments for the time are used.
This scientific field work lasted more than 70 years before reaching its finalization in 1815. A titanic undertaking based on an unprecedented method: triangulation, which allows for measuring distances with great precision by connecting fixed points such as towers, bell towers, and hills.
The inhabitants were invited to collaborate, particularly to name the mapped places. They sometimes did so with reluctance, because behind the "participatory" enterprise, they sensed the royal desire to better manage but also to better control the kingdom. For science, however, these new working methods allowed for the creation of the first complete territorial map of France, a true encyclopedia of the territory.

Prestige and posterity

This cartographic monument was distributed in a remarkable material diversity, from the ordinary copy to the most prestigious. The role of booksellers had great importance here. Some offered the map with a multitude of possibilities: coloring, canvas backing, binding, case, suspension rod…
The exceptional quality of some copies presented in the exhibition, including one on silk, reflects the eminence of their owner, such as King Louis XVI or Queen Marie-Antoinette. The gigantism of the Cassini map of France is only equaled by its longevity of use; the most striking being the creation of the Departments in 1790, for which it serves as a cartographic base. Other administrative uses persist to this day, such as those concerning rights related to mills. Its contribution to the current history of the environment or archaeology is just as essential. A scientific adventure with universal reach, an exceptional cartographic monument, the Cassini map of France became a model for Western cartography, whose methods it renewed.