Ups and downs... Mont Blanc is no exception. It is to precisely measure its altitude that a team of expert surveyors has climbed it every two years since 2001. This year, the expedition made up of twenty-two people, including Martin Fourcade, five-time Olympic champion biathlon, invited for the occasion, took place from September 14 to 17. The result will be revealed this Thursday, October 5, at 10 a.m. in Chamonix (Haute-Savoie), as part of a press conference relayed on social networks.
The highest point in Western Europe, measured with an accuracy of less than 10 cm, was at 4,807.81 meters in 2021, but in twenty years its altitude has varied from 4,810.90 meters, to the highest, measured in 2007, at 4,806.03 meters in 2019 (the measurements having been taken in bad weather, this was however not approved). If it experiences such variations, it is because its rocky summit, located at 4,792 meters, is topped by a cap of snow and ice of around fifteen to thirty meters thick subject to the vagaries of particularly violent and changing weather, which permanently shapes it, like a dune.
The weaker the winds and the greater the precipitation, the higher its peak and vice versa. It is also higher in summer, because storms are less frequent and the snow is less dispersed. In addition to its altitude, its position also changes with the winds. In 2009, the highest point was, for example, 34 meters further east than in 2003... And therefore more Italian than French.
It is to monitor its development that the Haute-Savoie Departmental Chamber of Surveyors sends a team to the summit every two years. Denis Borrel, president of the Commission for the measurement of Mont Blanc in 2023, explains to us the challenges of his mission.
Previously, measurement was done by triangulation. Surveyors from the IGN [National Institute of Geographic and Forestry Information] were stationed on different summits, including that of Mont Blanc, with theodolites [optical devices that allow horizontal and vertical angles to be measured], and Others, in the valley, measured the distances and deduced the altitude. This is how we arrived at this famous attitude of 4,807 meters that everyone learned at school
Since the 2000s, we have used centimeter GPS, much more precise than those fitted to our vehicles. They make it possible to receive signals from different satellite constellations – GPS for the Americans, Galileo for Europe, Beidou for the Chinese and Glonass for the Russians. All these satellites give their position in real time in the sky and allow us to have an exact measurement of the point where the sensor is located at the summit of Mont Blanc.
To find this summit, we move with a centimeter GPS along the practically flat ridge which is around a hundred meters long to locate the highest point. Then, we install a static GPS which will record measurements for an hour and a half to two hours. They are then transmitted to the IGN which makes slight adjustments to give us the exact altimetry which will then be reported on the maps.
To see if the snow and ice are melting, we use a mobile GPS that we plant in the snow at around a hundred points above 4,800 meters, which allows us to compare the volumes with those of previous years. This year, we achieved a great first since we also used a drone that we flew for fifteen minutes to scan an area measuring 100 meters by 100 along the summit ridge.
We did exactly the same survey as in two hours on foot but we recovered more than a billion points. Thanks to what we call the photogrammetry system, which consists of superimposing images to reconstruct the volumes, we were able to create a very precise model of the summit in three dimensions.
Climatologists and glaciologists said yesterday that Mont Blanc, a bit like a freezer, with its thirty meters of ice thickness, was protected from warming. How long will this stay true? This is what we are trying to determine. Scientists said it would take at least fifty years for a trend to emerge, but yesterday's predictions are no longer true today, as we can see elsewhere.
Since 2001, we have already observed an average decrease of 13 centimeters per year, but global warming is not necessarily responsible. The variation is not significant and, as climatologists say, it will probably take much more time to know whether or not global warming has an influence on this summit where the temperature, rarely positive, is most of the time. time around −15. It exceptionally rose this summer to ten degrees for two or three hours. It didn’t happen again, but – who knows? – maybe we will have a surprise on Thursday.
