A Matter of Centimeters

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Prior to the 1860 annexation of the Savoy region where Mont Blanc lies, Barre des Écrins–at 4,102 m (13,458 ft)–was the highest peak in France. The mountain is located in the French region of Provence-Alpes-Cote d’Azur (PACA) and was climbed for the first time by a group of British climbers in 1864. Today, Barre des Écrins– which, together with neighboring peaks Pic Lory and Dôme de Neige form the Massif des Écrins–remains a significant peak. It is the only peak higher than 4,000 m (13,120 ft) that lies entirely in French territory and represents a challenge for many alpinists who attempt to climb it every year.

More than just a mountain
In the summer of 2014, concurrently with the 150th anniversary celebrations of the first ascent of the Barre des Écrins, the Union Nationale des Géomètre Experts (UNGE) with his partners Geotopo and Geomesure organized an expedition to the mountain to make precise GPS measurements of the Massif’s summits. The measurements would determine the centimeter-level elevations of the three peaks and provide baseline data for an on-going campaign to assess Alpine uplifting over time. The expedition was open to surveyors of the PACA region.

Michel Baud, one of the organizers of the expedition recalls: "Due to the complexity of the approach to the peaks, the ascent of the Massif des Écrins is a huge undertaking even under normal circumstances, but doable with careful preparation. On our expedition we faced extreme weather conditions, the worst in recent memory. However, it was important for us to succeed: for the scientific goal, of course, but also for the personal challenge. We were all surveyors from the PACA region doing an expedition on the region’s mountains. Everything was highly symbolic."

The expedition
The expedition took place in two phases on separate climbs. Phase one involved the installation of small metal anchors near the tops of Barre des Écrins, Pic Lory and Dôme de Neige; these stainless steel rods–each 20 mm (0.8 inch) in diameter and 10 cm (4 inches) long–featured threaded ends for mounting the metal support rods and instruments during the second phase. Each anchor was to be placed in a hole drilled by a battery-powered pneumatic hammer as close as possible to each of the peaks.

The Phase one climb took place on June 26th, and involved six members, including engineers, geologists and surveyors. The group successfully installed two of the metal anchors on top of the Barre des Écrins and Pic Lory, but dangerous weather conditions blocked them from installing the third anchor on Dôme de Neige. This remaining anchor was installed on August 13th when four members–despite the wind and cold–managed to place it on top of Dôme de Neige.

Phase two began on August 27 and involved taking GPS elevation measurements to provide the baseline data as well as the first ever, centimeter-level measurements of the Massif. For the survey, the surveying team chose Trimble® R10 GNSS receivers and Trimble TSC3 controllers. Expedition surveyor Arnaud Ollivier explains: "In addition to being precise, we needed instruments that would be light, resistant and easy to transport: you can’t support any unnecessary weight in the conditions we faced." In order to gain more control over the calculation and greater precision, the team chose post-processing to calculate results. The Phase two group included about 20 participants and 5 mountain guides. The group was equipped with crampons, ropes, and carabiners. They carried laptop computers, four Trimble R10 GNSS receivers and three Trimble TSC3 controllers. On the first day, they were to install one of the Trimble R10 receivers on the roof of a mountain shelter located at 3,175 m (10,420 ft), just below the three peaks; this receiver would provide a control point for the measurements that were to be taken the day after, once the remaining three R10 receivers were positioned on support rods screwed into the metal anchors on the each of the peaks. To provide accurate geodetic reference, the survey was carried out using the Institut Géographique National’s (IGN) GNSS network in France.

The trek towards the mountain shelter was successful: after a day of climbing the sun-kissed Alps, the climbers reached it and, according to the plans, installed the Trimble R10 receiver on its roof. With a difference in altitude approximately 3,000 m (9,300 ft) between the IGN stations and Barre des Écrins, the team established the control point on the mountain shelter to reduce the chance of errors.

On August 28, at about 3:30 a.m., three roped parties set off from the mountain shelter in pursuit of their respective goals. Each party was directed towards one of the three peaks and was equipped with an R10 receiver, its support rod and a TSC3 controller. To avoid having to manipulate the devices in extreme weather conditions, the receivers had been configured via Web interface to collect data automatically at one-second intervals once the climb started. The TSC3 controller would ensure that the data acquisition would be reliably carried out.

After three hours of climbing, all three groups encountered extreme weather conditions. The groups climbing Pic Lory and Dôme de Neige were forced to stop halfway and return to the mountain shelter; snow and ice had made the climb too dangerous.

The third roped party was thankfully able to achieve its goal, reaching the metal anchor on which the Trimble R10 was to be mounted, at the summit on Barre des Écrins. They mounted the R10 onto the anchor and for about an hour collected data simultaneously with the R10 on the mountain shelter. "The team encountered significant challenges, but despite this and the other two groups having to turn back, they managed to reach the summit of the Barre. It was cold and humid, but the Trimble equipment performed excellently."

After collecting the data, the successful party headed back towards the mountain shelter. After a well-deserved rest, the entire group climbed down.

Precise measurements
Once back from the expedition, the team post-processed the data using Trimble Business Center software to confirm data integrity and produce preliminary results. They sent their data to IGN where the data was verified and corrected while taking into account variables such as atmosphere or variances of satellite orbits. It was determined that 4,102.10 m (13,458.3 ft) was the final result.

A new validation campaign is expected to take place in order to measure the two remaining peaks. More scientific expeditions are also planned in order to obtain the additional measurements necessary to assess the Alpine uplifting over time.

Despite the weather, the expedition was considered a success. In addition to having acquired the first measurement of Barre des Écrins to centimeter precision, the expedition turned out to be an adventure, brought awareness of the surveying profession to the general public and served an associative purpose by bringing together the elite surveyors of different PACA districts in service of a common goal. And last but not least, surveyors were able to provide scientists with a vital tool for monitoring Alpine movements in the future.

Fulvio Bernardini is a freelance technical writer, translator and editor based in Italy.

A 3.879Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE