Wednesday 27 November 2019

Tranchot and the mapping of the Rhineland

Monument to cartographer Jean-Joseph Tranchot on the Lousberg in Aachen

Anyone wishing to win over the affections of their valentine could do worse than to chaperone them to the summit of the Lousberg in Aachen. Admittedly, it doesn't have the same romantic ring about it as the Spanish Steps or Montmartre, but the view from the top of this humpbacked hill just outside the city centre is as enchanting as any in these whereabouts.
Indeed, the unbroken views offered from this lofty vantage point were not lost on early cartographers either, as the prominent obelisk on its summit bears witness.

The story of the obelisk dates back over 200 years, at a time when military aggression was good for mapmaking. After Napoleon invaded and then occupied the Rhineland at the turn of the 19th century, he demanded that detailed maps be made of his newly acquired possessions. The man chosen for the job was Jean Joseph Tranchot (1752–1815), a French army officer who had already carved out a distinguished career as a cartographer.

France and the birth of modern cartography 
Of all the nations in Europe during this period, France had taken the lead in systematised mapping. This was largely due to the Cassinis, a dynasty of accomplished astronomers and cartographers who, over the course of four generations from 1671 to 1793, occupied the position of Directeur général de l'Observatoire in Paris. Between 1750 and 1793, under the aegis of César-François Cassini (known as Cassini III), the first nationwide map of the country was produced in a total of 182 sheets at a scale of 1:86,400. It was achieved using the most advanced scientific principles of the day, namely astronomy and triangulation.

It was against this background that Jean Joseph Tranchot launched his career. His first surveying commission took him to Corsica where he was to help with the triangulation of the island of which France had taken control from the Republic of Genoa in 1768. After two decades serving on this project, Tranchot - who had singled himself for the great accuracy of his work - received a commendation from the French Academy of Sciences. Later he worked as a chief surveyor in the Méchain/Delambre measurement of the meridian arc between Dunkirk and Barcelona (1792-1798), which had the aim of establishing the exact size of the earth on which the new metric system would ultimately be based. 

The triangulation of the Rhineland 
So it was that Tranchot was tasked with mapping the newly conquered Rhineland in 1801. He set up his surveying operations in Aachen, the prefecture of the newly formed Département de la Roër, one of the four Départements réunis de la rive gauche du Rhin (on the left bank of the Rhine) stretching from Kleve (Cleves) in the north to Saarbrücken in the south. His first mission was to establish the exact geodetic coordinates of the highest point of the Lousberg, a prominent hilltop overlooking the town. From here the whole of the surrounding country could be triangulated and mapped. To do this he took meticulous astronomical observations from the summit over a period of 3 and a half weeks in the summer of 1803. The purposes of this was so that the Lousberg (264m) could be connected up to existing triangulation networks in France and the Netherlands and subsequently function as the starting point for the new Rhineland network.

From here, Tranchot created a system of 55 triangulation stations throughout the Rhineland. In the lower-lying areas, measurements would have been taken from church steeples. Those in Sittard and Erkelenz, which were in direct sight of the Lousberg, were amongst the first to be triangulated and they measured a distance of 28.12 and 36.59 kilometres respectively. However in other parts of the countryside, where higher points were covered in dense forest, such as in the Eifel, wooden beacons had to be constructed.

The French instrument of choice to triangulate was Borda’s repeating circle, a contemporary alternative to the theodolite. The instrument consisted of two telescopes mounted on a shared axis which could be rotated through 360 degrees. Using calibrated scales, the angle between the two – fixed on any distant object - could be measured. The telescopes could be swivelled and adjusted many times over so that multiple measurements could be made, so increasing the accuracy of the readings. 

The cartographic work 
Once primary triangulation in the Rhineland had been completed, topographic surveys could commence to map out the countryside in greater. This was done using plane tables, another piece of surveying equipment, consisting of a drawing board mounted on adjustable legs and used in the field for plotting measurements directly. The results of these measurements were recorded in ‘field maps’ (first at a scale of 1:10,000 and later at 1:20,000). Draughtsmen would depict variations in relief (i.e. slopes) by the use of hachuring where steeper slopes were represented by thicker, shorter ‘hatching’, while gentler slopes were represented by strokes which were thinner, longer and further apart. A very gentle slope or a flat area, like the top of a hill, was usually left blank.

For the Rhineland maps, cartographers were instructed to draw these hachures using the relatively new ‘oblique illumination’ technique, which introduced the principle of hill shading to cartography. In this way the terrain could be depicted in a more realistic fashion by showing how the three-dimensional surface might be illuminated from a point light source, for example, in the top left corner of a map. However, the cartographers of the day were not always properly acquainted with the technique. Others who had to work in haste did not always heed the requirements and there is said to be a lack of uniformity between the Tranchot maps.

The maps also included a cadastral element, so, for example, land was designated by its use: ‘B’ (Bois) indicated woodland; ‘T’ (Terres labourables) arable land; ‘P’(Prés) meadowland;  and so on.

After Napoleon’s abdication and subsequent exile to Elba in 1814, the French withdrew from the Rhineland. The topographic survey of the region continued under Prussian rule supervised by General Karl von Müffling from 1816 to 1828. The cartographic series dating from the whole of this period is therefore known as the Tranchot/von Müffling series. 

In all, 264 maps – each covering an area of 10 x 10 kilometres - were produced at a scale of 1:20,000 (later reduced to 1:25.000).

Part of the Tranchot/v. Müffling map of the Rhineland, sheet 76: Herzogenrath 

Snapshots of a bygone age 
The Tranchot maps paint a picture of the social and economic conditions of the pre-industrialised era in perhaps a more expressive way than any history book. Agricultural reform was still in its infancy, individual settlements had not yet expanded and new roads between major towns and cities had not been built. Through a comparison with modern-day maps it’s possible to understand the far-reaching changes that the landscape has undergone. On the other hand, in some remoter rural areas, it is surprising to see how much time has seemingly stood still for the last 200 years.

As for the Lousberg, the obelisk on the top of the hill tells its own story, one which captures the romance of cartography for map lovers like myself.

The triangulation point on the Lousberg was originally marked by a wooden signal. However, in 1807 a stone pyramid was erected on the spot and inscribed with various commemorative texts, not least proclaiming the accuracy of Tranchot’s geodetic calculations. The square base stood 3.6 metres high and the pyramid which rose above it reached a height of 8.7 metres. Later that year a lead casket was placed in the base containing 55 gold and silver coins and 13 bronze medallions celebrating key moments in the life of the Emperor Napoleon.
On 2 April 1814, after French troops had withdrawn from the Rhineland, the obelisk was torn down by Prussian soldiers hell-bent on looting the valuables contained in the lead casket. In the process, several stones were damaged, some toppling down the steep slope at the top of the hill.
Following this act of vandalism, in an act of generosity towards a fellow surveyor, Major-general von Müffling, the chief of general staff for the Prussian army in Niederrhein, immediately ordered the pyramid to be reconstructed, faithfully reproducing the texts that had been inscribed on the original pyramid, barring a eulogy in praise of the recently deposed Napoleon.

Happily, the monument still stands today to serve as a tangible record of the remarkable feats of cartographers of a bygone age. And if, while taking in the sweeping view from the top of the Lousberg, you were wanting to hold your heart-throb completely spellbound, what better way to do this than by telling them the captivating tale of Tranchot and his maps?

Further reading (in German):
Schmidt, R (1973) Die Kartenaufname der Rheinlande durch Tranchot und v. Müffling 1801 – 1828, Gesellschaft für Rheinische Geschichtskunde XII
Geilen Quirin (1991), Entstehung der Tranchot-Müfflingschen Kartenaufname und des rheinischen Kadasters in ‘Mit Wasser und Dampf’, Fehl, Kaspari-Küffen and Meyer, Meyer & Meyer 

This article was written as part of the Mapping the Mountains exhibition, organised under the auspices of the Dutch Mountain Film Festival #9, which takes place in Heerlen (NL) from 31 October 2019 to 29 February 2020.

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