If the km 0 of the French maps is at the foot of Notre-Dame, in Paris, the altitude zero is, she, in Marseilles, in the entrails of the tide gauge, a historical monument become watchdog for the Big Blue, of which she has taken the pulse for over 130 years.
When Mont Blanc is measured in September at 4808.72 m altitude, 1 cm less than in 2015, it is compared to the average level of the Mediterranean Sea. A level officially defined on January 1, 1897 by the tide gauge of Marseilles, in the cove Calvo, along the Corniche.
"If the first measurement dates from February 3, 1885, it is only after 12 years of observation that was officially set this level zero, "says AFP Alain Coulomb, engineer at the IGN, the National Institute of Geography and Forestry Information.
On January 1, 1897, a bronze rivet covered with platinum and iridium is sealed right next to the well where the copper float tide gauge "measures" the sea. At exactly 1,661 m above the water level. It is in relation to this "fundamental landmark" that are now calculated all the altitudes in the hexagon, with nearly 400,000 "terminals" already scattered across the country, and 1,500 new ones installed each year.
– Switzerland measured in Marseilles –
These measurements are made in France, but also in Switzerland, which, due to a lack of coastline, has been attached to Marseille to define its zero altitude. But not in Corsica, where the unevenness is calculated by the tide gauge of Ajaccio, nor in the departments or territories of overseas, which also have their own measures.
Classified historical monument in 2002, with the villa which hosts it, inhabited by a guardian until 1988, the tide gauge of Marseille is unique. If it is one of the few float mechanical tide gauges still to exist, "it is the only one in the world to understand an integrator, a totalizing part allowing to read constantly the average level of the sea over a given period", The Foundation for Heritage, which has just launched a call for donations to renovate the building, with nearly 10,000 euros already collected.
Second since 1998 by a system of notched wheels in the field of high-precision watchmaking. a digital camera, which calculates the sea level via a radar wave, the old Marseilles tide gauge remains useful for climatologists and scientists, as a witness to climate change.
– Climate change –
Only device to "measure" the Mediterranean for 130 years, it is thanks to the tide gauge that we know for example that the Mediterranean has risen 16 cm since 1987, and that it is 15 cm lower than the Atlantic Ocean … "It is an inestimable inheritance of Marseillaises and Marseillais, an exceptional tool and unique in the world", insists Renaud Muselier, the president of the region Paca, who has just pledged to finance the part of the 60,000 euros of necessary works that would not be covered by the call for donations.
If the astronomical tides, caused by the Moon, can vary the level of the Great Blue from 30 to 40 cm, the difference may exceed one meter with meteorological tides, due to highs or lows.
"With climate change, there is no longer any question of the usefulness of these measures. and the utility of the Marseilles tide gauge ", explains Alain Coulomb.