Pierres du Niton in Geneva

Thinking of working in Switzerland/already here? Are you enjoying the quality of life and the easy access to the mountains for winter sports and summer hiking? If so then read on if you ever wondered how the heights of the Swiss peaks were determined in the pre-global satellite positioning era. An engineering challenge with an interesting history traced back to Geneva's harbour.

If you look at a map of Switzerland made before 1902 and one made after you’ll notice that the country suddenly sunk by 3.26 metres, with the elevation data of all mountain tops, valley floors and plateaus shifting downwards by that amount.

The change can be traced back to the harbour of Geneva; stand on the Quai Gustave-Ador and look out over the water and you’ll see two grey rocks that poke out of the Lake.  These are the Pierres du Niton (Neptune’s Stones) and are erratics - boulders torn from their original location during the last age by the Rhône glacier and deposited on what is now the lake bed when the ice retreated at the end of the Alpine Würm glaciation some 19,000 years ago.

Their geology and composition suggest that they came from the area around Mont Blanc and their large size has prevented them from being washed away through the centuries.  They even survive the Tauredunum event when a 17-metre high tsunami swept around the lake in 563AD triggered by a huge landslide at the eastern end and that was still an estimated 8-metres high when it reached Geneva so that it came over the city walls causing destruction of property and loss of life.

Marius of Avenches wrote about the catastrophe in his Chronicle,

“It swept away with fury the Bridge of Geneva, the mills and the men; and, flowing into the city of Geneva, caused the loss of several lives.”

Although two rocks are visible above the water there are actually twenty-four large blocks in this location and being an unusual and distinctive feature, they were chosen by Guillaume-Henri Dufour as the elevation reference point for his Dufour maps of Switzerland between 1845 to 1864. 

Dufour had an impressive army career, trained as an engineer he rose to the rank of General and led Swiss Confederation forces in the civil war of 1847 that ultimately led to the consolidation of a Swiss federal state.  Dufour presided over the First Geneva Convention in 1864 that included the creation of the International Red Cross and was the founder of the Swiss Federal Office of Topography, arguably the great-grandfather of Swiss map-making and a career woven through the birth of the confederation.  His equestrian statue stands in the Place de Neuve, Geneva on the traffic island between the Parc des Bastions and the Musée Rath.

Dufour wasn’t the first to take a keen interest in the rocks however; in the 17th century it was noted that square holes had been cut into the top of the larger of the stones using tools from the Bronze Age and it seems likely that the distinctive rocks would have held sacred significances and perhaps been involved in religious ceremonies.  There are also circular man-made depressions called cupules of unknown origin and purpose.

Durfour added a nine-and-a-half-foot long brass rod to one of the stones in 1820 to act as a water level gauge and allow the fluctuations in the lake’s height to be studied.  The stones made an obvious choice as a stable feature, close enough to shore that the graduated rod could be read by telescope.  The measurements were used in the December 1843 edition of the Society of Physics and Natural History of Geneva as evidence that new machinery installed in the city to supply water to public fountains was not affecting the level of the lake.  Dufour also fixed a brass plaque to the stone which was to be used as the reference point for all Swiss elevation data.

Since Switzerland is a land-locked country, the crucial step of determining the height above sea level of this brass plague on the Pierres du Niton proved challenging.  Various attempts from 1820 (the canton of Geneva actually marked the bicentennial of the efforts) through to 1879 recorded it as being between 376.52 metres and 376.86 metres above the sea, using the tide gauge at Marseille as the reference and earlier work by Napoleon’s engineers who had drawn up trigonometric surveys from Strasbourg and the Atlantic Coast to the Swiss border.

It wasn’t until 1902 however, that Jakob Hilfiker settled the matter by using a number of different European tidal gauges to fix the Niton reference point as 373.60 metres, sinking the country with one stroke of his pen and providing the final, definitive value for Swiss map making.

And to this day, the Pierres du Niton remain the reference horizon for the système géodésique de référence altimétrique en Suisse (and for Lichtenstein also) – all altitudes are measured in relation to the height of the stones in the lake which is called the Repère Pierre du Niton (RPN) and an earthquake or an even bigger tidal wave notwithstanding, the country’s mountaintops now remain fixed and firm.