Cartography in the service of the monarchy: the Cassini map

Cartography in the service of the monarchy: the Cassini map

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Title: Cassini map, detail of sheet 125, Cherbourg.

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Technique and other indications: Watercolor engraving

Storage location: Manche departmental archives website

Contact copyright: © General Council of Manche, departmental archives

Picture reference: Cassini map, sheet 125 (detail: Cherbourg)

Cassini map, detail of sheet 125, Cherbourg.

© General Council of Manche, departmental archives

Publication date: September 2013

Manche departmental archives

Historical context

The Cassini map, which it would be more accurate to name the Cassini map since it takes its name from a family of four Italian geographers who settled in the kingdom at the end of the 17th centurye century, is the first major cartographic company covering the whole of France. It consists of 180 sheets, the surveys of which, begun in 1756, were completed in 1789. Intended to be marketed to a wealthy public, some editions have also been watercolored.

Image Analysis

The document reproduced here is a detail of sheet 125 (Cherbourg), the twenty-second having been the subject of a publication.

The Cassini map owes its extraordinary precision to the triangulation method, requested for its development. This method consists of measuring the distance between two points B and C and then, from a reference point A, measuring the angles of lines BA and CA. The geographer then simply needs to apply the trigonometry formulas in order to know the distances BA and CA. This method was used by the Dutch Snellius in 1615, then by Abbé Picard in 1670 and, finally, from 1683 to 1718, for the measurement of a large meridian stretching from Dunkirk to Perpignan. The corners of each sheet of Cassini's map show the distances in toises from the meridian of Paris and its perpendicular (which joins Saint-Malo and Strasbourg). Besides this main triangulation, each board has nearly 300 landmarks (buildings, heights), useful for establishing a secondary triangulation.

In fact, the mentions made on the card are numerous and varied. It thus contains information relating to the relief (forests, marshes, rivers, ponds), communication routes (roads, canals, bridges), administrative and religious organization (the limits of province, diocese, parishes) or to civil and religious buildings (churches, abbeys, priories, mills, castles). Military information also appears on the document, such as battles won or lost, or even certain battlefields: the map of Cherbourg thus mentions, not far from the city, the camp built in 1756, in the midst of the Seven Years' War, for the defend against English incursions.


The development of the Cassini map is indicative of the Enlightenment's enthusiasm for cartographic work and the development of communication networks. However, this development is closely linked to the progress of absolutism because, to manage the kingdom more effectively, distribute and levy taxes, draw roads and defend its borders, the royal power needs to know well the territory of which it has control. charge. From 1740, the engineers of the Ponts et Chaussées thus produced the so-called "Trudaine" atlas. It was also in the continuity of this movement that, under the Empire, the law of September 14, 1807 ordered the establishment of cadastral plans for fiscal reasons.

After Cassini's map, the geographical coverage of French territory was left to the military, proof of the highly strategic interest of such an achievement. A general map of the staff was thus drawn from 1816 to 1866. In 1940, however, the geographical service of the army became a civilian body: the National Geographic Institute (I.G.N.).

The Cassini map is still used today by many researchers, amateurs and professionals, attached to the study of toponyms, archeology, historical geography or even the history of the environment.

Read also the file dedicated to the card on the site

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  • Normandy
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Monique PELLETIER, The Cassini Card. The extraordinary adventure of the map of France, Paris, Presses of the National School of Bridges and Roads, 1990.

Monique PELLETIER, The Cassini Cards. Science at the service of the State and the regions, Paris, Committee for Historical and Scientific Work, coll. "C.T.H.S. Format ”, 2002.

To cite this article

Jérémie HALAIS, "Cartography in the service of the monarchy: the Cassini map"

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