The Cassini map

The King (examining a map): With their precious works, these gentlemen of the Academy are taking more territory from me than all my enemies put together !

It was the early 1680s, and Louis XIV was giving an audience to the eminent members of the Académie des Sciences, founded a few years earlier by Colbert. After several months of travelling along the coasts of France, two of its members, Abbé Jean Picard and Philippe de la Hire, had just completed the new map of the cost lines of the kingdom and presented it to the King. They showed him a France that was much narrower than that depicted on the previous maps. It was this “narrowing” that gave rise to Louis XIV’s remark. However, the king continued to finance the work of the Académie and its most important facility, the Paris observatory, which had been run since its foundation by Jean-Dominique Cassini, the ancestor of a veritable dynasty that reigned over French astronomy and a major part of cartographic production for more than a century.

The map by Picard and La Hire marked an essential break in the history of French cartographic production. Once the perimeter of the kingdom had been determined, cartography could no longer be based on the simple addition of local knowledge, as had previously been the case. The pieces of the jigsaw had to fit into a specific framework. To establish this, the authors followed a method developed by Cassini by observing the occultation of Jupiter’s satellites.

At the same time, Picard had a more ambitious project in mind. After contributing to the development of new measuring instruments, he proposed to renew the map of France by means of an operation that was unprecedented at the time: a triangulated survey of the whole of the Kingdom. While maps had hitherto been drawn up on the basis of distance measurements, Picard proposed measuring angles using long-distance sights. Initial work began in the mid-1680s, but it was a long-term project that neither Picard nor Cassini were able to complete. It was not until the following century, in 1744, that the entire kingdom was covered. During this period, Picard and Jean-Dominique Cassini died, but Jacques Cassini, Jean-Dominique’s son, took over from his father in 1712 – and ran the Observatory until his death.

From then on, French cartographic production was framed – in the truest sense of the word – by the great triangles of the work initiated by Picard sixty years earlier. From then on, cartography would be drawn up by division of the territory, on the basis of second- and third-order triangulations. The foundations for a completely new map had been laid, and all that remained to be done was to measure secondary angles and carry out topographical surveys.

Only three years after the completion of the triangulation and well before the publication of its results, César François Cassini, grandson of Jean-Dominique and son of Jacques, convinced King Louis XV to provide financial support for the preparation of a detailed map of the Kingdom. The first 2 sheets – Paris and Beauvais – were published in 1756, and each year thereafter brought its own batch of new sheets. The result was the famous Cassini map. This was the first map of France to be based on geodetic survey. To top it all off, César François succeeded his father as director of the Observatory in 1756.

The map was divided into 181 sheets engraved in intaglio at a scale of one line per 100 toises – one inch on the map corresponds to 339 inches in the field. This reduction ratio allows to show urbanized areas from large towns to small villages, castles, the road network, watercourses and hydraulic works. It also distinguishes between woodland and undeveloped land by means of specific motifs. On the other hand, the poor knowledge of sea levels – the respective altitudes of the Mediterranean, the ocean and the North Sea are unknown – the weakness of the levelling instruments available and the lack of resources mean that it is not possible to survey altitudes. As a result, the main slopes are represented by fairly rough hatching.

According to his initial plan, Cassini intended to publish ten new sheets each year. To achieve this, he needed to have as many teams of two topographers each and the necessary instruments. It was not possible to assemble such resources; Cassini also struggled to recruit engravers. From the mid-1750s, the logistical difficulties were compounded by the kingdom’s financial problems; the king stopped supporting the venture. Cassini then developed a private company with around fifty partners, including the prestigious Madame de Pompadour. By combining the financial contributions of the partners with those of the provinces and counties (Guyenne, Bresse, Brittany, etc.), those of the subscribers and the income from the sale of single sheets already published, the company’s accounts balanced out and it managed to publish eight new sheets a year at the turn of the 1750s and 1760s.

César-François Cassini died in 1784, and the company was taken over by his son, Jean Dominique. According to the balance sheet he drew up after the Revolution, there were still fifteen sheets to be published, but the new chamber did not allow him the time. Even before the Revolution, military surveyors took a dim view of the fact that cartography – an eminently strategic field – was being managed by a private company. In 1893, in order to protect the confidentiality of the information it contained, the Convention confiscated the map – the manuscripts, the copperplates and the last copies – in favour of the War Office for mapping which took on the task of completing, supplementing and publishing all the sheets.

This transfer did not harm the map insofar as it gave it the status of a strategic document and several sheets were completed by the War Office for mapping. In addition, from the early 1890s, several private publishers, the most important of which were Louis Capitaine and Pierre Gilles Chanlaire, published maps derived from the Cassini’s one at smaller scales. These publications were in turn taken up by other publishers, giving rise to a multitude of maps, both topographical or thematic, which dominated the French cartographic scene in the first half of the 19th century.

From 1832 onwards, the publication of the first sheets of the new detailed map – “La carte d’État-Major” -, which depicted the relief with the greatest possible precision, downgraded the Cassini map. Cassini’s map hours would have been numbered had it not been for the difficulties encountered by the General Staff, which delayed the completion of his map until the early 1860s. Until then, the Cassini’s map was in use for the regions still to be covered by the new map.

Jean-Luc Arnaud