Drone, lack of oxygen and extreme weather: how do we measure Mont Blanc?

“It interests all French people since it is the only altitude that the general public knows today and that we learned about in our school books!” Denis Borrel is a surveyor at Viuz-en-Sallaz (Haute-Savoie). After three expeditions since 2017, he became the president of the commission for the measurement of Mont Blanc in 2023, which took place in September. The updated altitude of Mont Blanc, Western Europe’s highest peak, is due to be revealed on Thursday October 5.

>> Three things to know about the altitude of Mont Blanc, which has lost more than three meters since 2010

Mont-Blanc has been measured every two years since 2001. The last measurement dates from 2021, with a precise altitude of 4,807.81 meters. Between 2010 and 2021, the roof of Europe fell by more than three meters. On average, it has lost around ten centimeters per year since 2001 according to surveyors.

These actually measure the snow cover at the summit of Mont-Blanc, which changes depending on the wind. These measurements also provide reliable data for experts, such as glaciologists and climatologists.

Four days shipping

This time, the expedition left Chamonix on September 14 for a duration of four days of ascent and surveys. The 22 participants, including seven guides, left with three GPS devices in their pockets. The first measurement stops at the summit of Mont-Blanc in order to know its highest point.

A second GPS, static on the summit, takes measurements for two hours, with data then refined, to the nearest centimeter, by the National Geographic Institute (IGN), with a margin of error of ten centimeters. A third, mobile GPS surveys the summit to obtain measurements of the ice cap. Comparing them to previous measurements makes it possible to analyze the evolution of the ice volume.

GPS can provide accurate data in just two hours. According to Denis Borrel, it would not be possible to collect this data without these new technologies, because it would be necessary to stay longer at the summit, while enduring the extreme cold (between -10 and -30 degrees).

“This year, we were more under the sign of a feminization of the expedition, explains Denis Borrel, president of the commission for measuring Mont-Blanc in 2023. The profession is becoming more feminized, and there were five female surveyors with us out of the 22 participants to climb to the top.”

A unique measurement by drone

This is a first in France: this year, experts took a drone to take aerial measurements and obtain a 3D model of the cap at the summit of Mont-Blanc. “A point was taken every two square centimeters on an area of ​​100 meters by 100 meters at the top”, says Denis Borrel.

“The drone flight allowed us to produce a billion points in five minutes!”

Denis Borrel, expert surveyor

at franceinfo

This drone measurement required authorizations from the prefecture, the town halls of Chamonix and Saint-Gervais, but also from the Directorate of Security and Civil Aviation, with a predefined flight plan. These authorizations are granted for a quarter of an hour of flight. “This type of flight cannot exceed 20 minutes, which is the autonomy of the batteries used today on these drones.”

Using the drone, experts could in the future also analyze the development of the snow volume below the summit. “We would like to go down to 4,700 meters to ultimately see if there is also a reduction in the volume and thickness of possible snow in these lower areas. Today, we only compare data that is acquired at above 4,800 meters”explains the surveyor.

Mont Blanc has not been measured for 40 years

Before the launch of these expeditions in 2001, no measurement of Mont Blanc had been made in 40 years. “The latest IGC cards [de l’Institut national de l’information géographique et forestière] dated from the 1950s and 1960s”, Denis Borrel is surprised. The first records date from the 1750s.

“At the time, they placed a prism at the summit on a cane and from the bottom of the valley, several people aimed at this famous prism. By triangulation and geometry, they were able to deduce the coordinates of the summit, explains the surveyor. Then, they mounted different optical triangulation instruments [par laser]even barometers, which calculated the atmospheric pressure and therefore deduced, with the temperature, the altitude of Mont-Blanc.”

The development and intensification of expeditions from 2001 can be explained, in particular, by the awareness of global warming. “Before, they believed that it was not important to know the precise altitude of Mont-Blanc because global warming was not necessarily current. However, today, we see that the glaciers are melting higher and higher in the valleys, that pockets of water are being created and that the permafrost is also melting higher and higher.”

Can we justify the reduction of around ten centimeters per year due to global warming? “The link is the scientists who will give it to us, the glaciologists and meteorologists, who will analyze this data”, answers Denis Borrel. All measurements are available on the 4810.eu website.

“Tears” and 30% less oxygen at the summit

These experts are the only ones to spend so much time at the summit of Mont Blanc. “It’s exhilarating, admits Denis Borrel. Already, on arrival, there are hugs and hugs. There may be tears from some who have never been on the roof of Western Europe.”

But the tears are quickly dried because you have to go quickly to take the measurements.“The emotional part is quickly erased by the technical part. We are there for a given time and we have a whole battery of measurements to carry out, knowing that we are still subject to hypoxia at this altitude: there are 30 % less oxygen so reflexes and thinking are impaired.”

The challenge of this 2023 measure was the use of the drone at 4,800 meters. “We have a lift which is much less at this altitude. So we must check that the batteries are charged to the maximum, that all the parameters are ‘ok’ so that the lift of the drone is sufficient to take ten minutes of readings at this altitude. altitude.”

A quarter of an hour to 2.5 hours at the summit, depending on the weather

Physical training and material preparation are not enough for the success of the expedition. Weather conditions, sometimes unpredictable, can reduce the time spent at the summit of Mont-Blanc. “When we went up in the spring, there was such a wind that the conditions started to become a little more drastic, remembers Denis Borrel. So we stayed at the summit for a quarter of an hour.”

This expedition in September fortunately took place with ideal weather. The 22 participants were able to stay up to two and a half hours at the summit.

An Olympic champion with the experts

For this expedition, the expert surveyors were accompanied by Martin Fourcade. “For the mountain enthusiast that I am, Mont-Blanc remains an inexhaustible source of fascination, imbued with its mythical dimension, confided the five-time Olympic biathlon champion, before the expedition. As a high-level athlete, I spent a decade evolving in a world governed by numerical data, between stopwatches, distances and speeds… Thus, the exceptional opportunity to participate in the 2023 measure touches me deeply .”