From a distance, they look like the backs of whales, rising above the water, but up close, the two objects in Lake Geneva’s basin are revealed to be grey boulders. The Pierre du Niton and Pierre Dyolin are fixed landmarks in Geneva’s cityscape; their pronounced appearance in shallow water provides a natural orientation point.

However, the two granite rocks have not always been such an unwavering presence in the lake. They were formed over 300 million years ago and underwent a significant migration: in the last ice age, the Rhone glaciers brought them from the East Mont Blanc massif to their current location in Lake Geneva. The Pierre du Niton and Pierre Dyolin are boulders.

The rocks surrounded by water drew the attention of the inhabitants in the Bronze Age. A 20 by 40 cm large and 20 cm deep rectangle on the back of the Pierre du Niton suggests that the stone was used as an altar by Bronze Age Celtic tribes.

Conflict at Lake Geneva

In 1820, the inhabitants inscribed the Pierre du Niton once again. This time, however, was not about worshipping the gods but rather about a conflict within the Swiss Confederation: the Cantons of Vaud and Valais accused poles, dams and industrial equipment of blocking the flow of the Rhone in Geneva so much that it repeatedly caused flooding on the shores of the lake.

In order to settle the dispute, Guillaume Henri Dufour – an engineer for the city and Canton of Geneva – decided in 1820 to precisely monitor the water level in the Lake Geneva basin. One of the measures used was to install a circular water level gauge with a diameter of 85 mm on the back of the Pierre du Niton, which was intended to make it easier to monitor the water level at the point where the Rhone flowed into the lake.

Becoming the height reference point – with a little help from France

When the Pierre du Niton was given its first absolute height in the late 1820s, this was a by-product of a major undertaking. Although it was possible to determine height differences between individual points within Switzerland using angle measurements, their height above sea level was unknown. Because Switzerland is a landlocked country, the sea level from abroad – more specifically from France – had to be applied to Switzerland.

Back in 1803/1804, Napoleon’s geographic engineers measured a triangulation network from Strasbourg over the Jura mountain range and Alpine foothills to the “Les Voirons” mountain range east of Geneva, allowing them to determine several heights in Switzerland. In the 1820s, France also measured a triangulation chain from the Atlantic to the western border regions of the Confederation. These measurements helped to determine some more heights of various points in Switzerland.

As part of these efforts, the French geographic engineer Charles-Marie Filhon (1790–1857) also determined a height for the water level gauge of the Pierre du Niton in the late 1820s, which was published in 1832. According to Filhon’s calculations, the circular disc stood 376.55 m above the Atlantic sea level. With the absolute height, Filhon added another meaning to the boulder, altar and level indicator, making it one of the few points in the Confederation for which an absolute height had been reliably determined.

However, the rock in the Lake Geneva basin still had to wait another several decades to take up its special position as the Swiss reference height which we continue to use today. For the elevation levels of the Dufour map, the mean value of two French height measurements of the Chasseral was still used as the reference height (1609.57 m above sea level). This was established in 1840 by astronomer and geodesist Johannes Eschmann.

Elevated status for the height starting point

The early Swiss federal state evolved rapidly. The boom in the economy, culture and tourism was echoed by a call for spatial knowledge of the highest accuracy – and this included the height and depth of land surfaces. Founded in 1861, the Swiss Geodetic Commission of the Swiss Academy of Natural Sciences committed itself to pursuing the matter of height from 1864 and decided to perform a Nivellement de Précision (precision levelling). Compared to height measurements by triangulation, which had been commonly used in Switzerland up until this point, the technique of levelling was much more accurate.

The bronze mark of the Pierre du Niton functioned as the starting point in the first Swiss levelling system, replacing the triangulation point on the Chasseral and becoming a key point for national surveying. In 1879, Dufour’s successor Hermann Siegfried (1819–1879) officially determined the point to be 376.86 m above sea level. Controversially, he based this on Eschmann’s Chasseral level, the accuracy of which had already been questioned in Dufour’s time.

The value of 376.86 m above sea level was used as a reference height for all levels and contour lines of the Siegfried Map, the successor of the Dufour Map. However, taking into account the points where neighbouring countries met the sea, in particular the French levelling measurement from Marseille, the geodesist Jakob Hilfiker (1851–1913) established a new, more accurate height indication for the gauge on the boulder in 1902. The value resulting from his calculations was 373.6 metres above sea level. This “new horizon” has been used since 1910 as the established statutory initial height and, since the 1930s, as the reference height for the national map which replaced the Siegfried Map.

Boulder, water level gauge, reference height

The bronze circle on the Pierre du Niton evolved from a water level gauge (1820) into the horizon marker for Switzerland (1879). The significance of the height starting point, which it still has to this day, became apparent in the transition from the Siegfried Map to the national map. With the change from the old to the new horizon, Switzerland as a whole was lowered by a good three metres.

In the last 200 years, striking landscape features have had an almost invisible but extremely powerful impact in their role as geodesic points. The Pierre du Niton, which was formed some 300 million years ago in what is now the Mont Blanc massif and has served as an altar to the Celts and a water level gauge in an attempt to settle a dispute between the inhabitants of Lake Geneva, is perhaps the most impressive testimony of this development.

In the anniversary publication “Die Schweiz auf dem Messtisch. 175 Jahre Dufourkarte” the swisstopo geodesist Andreas Schlatter explains in detail how the heights in Switzerland were established. In addition, you can also expect six further exciting articles in this book on the relationship between maps and history.

Order the book here