Tuesday, August 2, 2016

Cassini Maps of France

One section from the Marseille sheet of the 18th century Cassini map of France..

Here's another very useful resource that I came across just this week -- the Cassini map of France. The map divides France into 181 sheets. Point at a sheet and a short version of the sheet name appears.

Select a sheet (just click on it) and a new window appears that shows just that one sheet. Each sheet contains multiple rectangular sections (often 21 in number).

The Orleans sheet of the Cassini Map. 
The sheet window includes tools to zoom, pan, reset, and a full screen view. Zooming in fully gives a fairly readable map. Once I had fully zoomed in, I used the snip tool to copy part of the map.

The first maps of France that used a measuring apparatus were made by the Cassini family during the 18th century. There maps were made on a scale of 1:86,400 (one centimeter on the chart corresponds to approximately 864 meters on the ground). These were the first maps made using geodetic triangulation. It took more than fifty years to complete and four generations of the Cassini family were involved in their production. These maps, known as "Cassini Maps" or "maps of the Academy," are still referenced by geographers, historians and genealogists.

These maps were primarily the work of César-François Cassini de Thury (Cassini III) and his son Jean-Dominique Cassini (Cassini IV. The adopted scale is one line to 100 toises, or 1:86,400 (the measuring apparatus contained 864 lines). The toise was a unit of measure in Ancien France which was equal to 6 pieds (feet) or about 1.949 meters. Map surveys were carried out between 1756 and 1789 and the 181 sheets composing the map were published from 1756 to 1815.

The Cassini maps don’t pinpoint dwellings or the boundaries of marshes and forests; however, the level of precision of the road networks is such that the drawn roads correspond almost exactly to satellite photographs. The maps are still consulted by historians. The maps accomplished three goals.

  1. Measure distances by triangulation to ensure the exact positioning of locations.
  2. Measure the kingdom and determine the number of parishes, cities, and villages.
  3. Depict landscape features that are not subject to rapid or frequent change.

The site includes several versions of the Cassini map sheets both color and black and white.  

If your interest is Britain rather than France, you should be familiar with the excellent Ordinance Survey maps of Britain which were produced after the Cassinis began their mapping. You can even get earlier editions from Ordinance Survey.

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