A bronze rivet sealed in 1897 in Marseille defines France's altitude zero, influencing 400,000 markers across the country and connecting to Switzerland, while Corsica uses its own reference.
Origin of French altitude measurements
Each altitude marked on a mountain sign, an IGN map, or a hiking marker in mainland France starts from a bronze rivet topped with platinum and iridium, sealed on January 1, 1897, in a villa on the Corniche in Marseille. This rivet is located 1,661 meters above the zero level of the sea, defined from observations made between 1885 and 1897.Reference point and its significance
Nearly 400,000 altitude markers in mainland France are calculated from this reference point. Switzerland is connected to this zero, while Corsica has its own reference level, established in Ajaccio. The zero itself is a sea level, defined from observations made between 1885 and 1897, but it is not engraved anywhere and remains difficult to access, at the bottom of a well. The rivet therefore plays the role of a physical point, one that surveyors can aim at and connect to everything else.Definition of the zero level
The zero is not a simple arithmetic average. Charles Lallemand, who became director of the Service du nivellement général de la France in 1891, and his engineers traced the curve of the mean sea level, a hyperbola. They adopted its horizontal asymptote, towards which the level seemed to tend. The result remains extremely close to the average of the readings taken between 1885 and 1897. The choice is conventional: another location or another period could have given a significantly different result.Physical structure of the reference point
The site consists of two buildings: a sturdy shelter for the equipment and a house for the warden. A warden lived there until 1988. Since 2002, the entire site has been classified as a historic monument. Under the shelter, a subterranean chamber houses the fundamental reference point. The rivet is sealed in a cylindrical granite block, itself embedded in the rock of the promontory. The IGN describes these conditions of stability and preservation as very favorable.Continuation of the reference system
From this base, surveyors have calculated all the altitudes of the Hexagon, step by step. According to figures from the IGN reported by AFP in 2018, there are nearly 400,000 markers, and 1,500 new ones are added each year, about 4 per day. The public rarely enters there: 200 visitors during Heritage Days. A country without a sea has no sea level to measure, and Switzerland has therefore connected itself to the zero of Marseille.Historical and international context
According to the IGN's sheet, French engineers probably hoped that their observatory would one day be used to establish a single zero for Europe. Corsica, on the other hand, has its own zero, taken in Ajaccio. Its system, the NGF-IGN78, is based on the tide gauge in Ajaccio, while the NGF-IGN69 of the mainland is based on that of Marseille.Long-term measurements and sea level rise
The first continental network, established by Lallemand from 1884 to 1922, set its origin based on the Marseille observations. The tide gauge has not stopped working since 1897. Its recording rolls, 9 meters long, covered each month, and twelve per year were sent to Paris, where they have all been digitized. Since 1998, the data transmitted to the international service that centralizes sea levels come from a digital tide gauge. The series covers more than 135 years, practically without interruption.Sea level changes and reference point stability
Alain Coulomb, former IGN engineer and president of the Friends of the Tide Gauge Association, mentions a rise of almost 20 centimeters since 1885. According to him, the rate would have changed from about 1.2 millimeters per year last century to 3.5 to 4 millimeters per year in recent years. Other estimates range around 16 to 18 centimeters, depending on the period considered.Stability of the reference over time
The zero does not follow this rise. During the restoration of the base network between 1962 and 1969, the IGN retained the origin defined by Lallemand. The rivet has not moved either, and the gap separating it from the sea is closing at the rate of the rise: with 16 to 20 centimeters gained since 1897, it would now be around 1.5 meters. The site also has a permanent GNSS station. An altitude indicated on a mountain pass does not get corrected because the Mediterranean has gained a few centimeters: the 1897 zero remains the reference.
