The 2019 Nepalese survey of Mount Everest, utilizing advanced tools and techniques, yielded the most precise elevation to date of the world’s highest and most famous peak.
By now, we have all probably heard that Mount Everest has a new official elevation, announced jointly on December 8, 2020 by Nepal and China. They coordinated the results of their respective 2019 and 2020 surveys of the iconic mountain to agree upon the new elevation— 8,848.86 meters or 29,031.69 feet (height above sea level). This is a little less than a meter higher than the previous reported elevation. Differences could be due to tectonics, reference framework elements, and differences in the definition of “the top” (e.g., is it the top of the rock underneath the ice or top of the ice?). But more likely, it is the modernized surveying methods and technologies utilized in these surveys.
What we do not hear much about in the flurry of news surrounding the announcement are details on the survey, the surveyors, the challenges they faced surveying in such extreme conditions, and the amazing technologies they used.
Here we dive into the surveying and geospatial side of this news, and include a first-hand account of the climb and survey from the lead surveyor on the Nepal Survey team.
3:00 a.m. on May 22, 2019
The team from the Survey Department of Government of Nepal reached the summit of Mount Everest. The darkness, wind, brutally low temperatures, exhaustion, and limited oxygen supplies stacked the odds against their completing the GNSS survey and related measurements in the limited time window. Yet the team — Chief Survey Officer Khim Lal Gautam, Survey Officer Rabin Karki, lead Sherpa Tshiring Jangbu and two additional Sherpas — prevailed, a testament to the tenacity of the surveyors, the support of the Sherpas and the toughness of their instruments.
But hadn’t the mountain been measured many times before? There have been dozens of re-measurements since 1852 when, during the Great Trigonometric Survey of India, “Peak XV” (as it was known at the time) was identified as the world’s tallest mountain. The quest for a more precise elevation has seen resurveys with a progression of instruments and methods ranging from trigonometric to GPS in the early 1990s and later GNSS. But why remeasure again? Why commit tremendous resources and put surveyors in harms’ way?
As fate would have it, a natural disaster served as catalyst for the new survey. That catastrophe, along with other drivers for the geodetic survey of the region, provides context for the decision to obtain new summit measurements.
The dynamic range
In April 2015 during the peak of the climbing season, Nepal experienced a deadly 7.8 magnitude earthquake, suffering substantial damage, injuries, and loss of life. While recovery activities may continue for years, the country has made tremendous progress restoring what was damaged, leading to vibrant national growth. Post-quake questions arose about the possible effects on the region’s geology — and the elevation of Mount Everest. The mountain felt the impact of the quake, including a deadly avalanche.
Understanding the effects of the quake was certainly a driver for the recent Sagarmatha (Everest) Height Measurement project undertaken by the Survey Department of Nepal, but not the only one. Other technologies have not detected significant height changes in the mountain, though there is horizontal displacement due to tectonics. Satellite-based InSAR (synthetic aperture interferometry), like this study by the USGS, shows there is little vertical movement, though it does in the horizontal. Therefore, the new elevation was likely due mostly to the improved methods, technologies, and modernized geodetic reference elements. A more compelling driver was that Nepal has long desired a broader understanding of the region’s geophysical dynamics and a modernization of the regional geodetic reference infrastructure.
There is also a matter of national pride: The Himalayas are one of the youngest and most dynamic of the world’s major mountain ranges. Rising from the collision point of the Indo-Australian and Eurasian tectonic plates, the Himalayas span an area larger than the state of California. Nepal is home to not only the iconic Everest, but also seven more of the world’s fourteen “Eight-thousanders” — mountains higher than 8,000 meters (26,247 feet). The majestic peaks are a source of national pride, identity, and economic activity for Nepal. Yet history has seen the focus on the most famous mountain in this range co-opted by outsiders — but no more. This project was also the first time the Nepalese government had measured the elevation themselves. All previous measurements were expeditions from other countries. With this survey, Nepal aims to become the foremost authority on the geodesy of their own national treasures.
The official title for the survey, a multi-faceted geodesy project spanning 2+ years, is the “Sagarmatha Height Measurement.” The mountain carries many names: Peak XV, then “Everest,” for Sir George Everest, the British surveyor general of the Great Trigonometrical Survey (who expressed misgivings about having the peak named after him). But there are also other local names, such as the Nepali Sagarmatha and Chinese Zhumulangma. While the word Sagarmatha, roughly meaning “head of the earth touching heaven,” already existed in Sanskrit and Nepali, it was not suggested as a Nepalese name for the peak until a 1930 essay by Baburam Acharya, a Nepalese historian and scholar. The name did not catch on immediately; in fact, there was pushback, and he was nearly deported for his suggestion. Since then, however, the name has gained broader acceptance.
If there is disagreement on the name, so too the official elevation. The Great Trigonometric Survey established the elevation as 8,840 meters (29,002 feet), with subsequent surveys refining this to the broadly-accepted official elevation of 8,848 meters (29,029 feet) — this from a1955 Indian survey and backed-up by a 1975 Chinese survey. But there is another controversy: do you measure to the highest point of the mountain, or the top of the ice/snow, which can vary by meters seasonally. The advent of ground penetrating radar (GPR) — portable enough to carry to the summit — has provided the ability to measure both. The Nepal survey included GPR measurements on the summit as well.
As new technologies have been added to the mix — satellite imaging, lasers and GNSS — there should be more consensus on the elevation, but this has not been the case. Measuring the elevation of Everest is much more complex than most people would imagine. For any attempt to establish a “height above sea level,” geodesists and surveyors require a refined gravity model and geodetic reference infrastructure. The Nepalese geodetic survey of the region was designed to provide it all.
The network
The ability to derive a high-precision elevation, or height above “sea level,” is subject to a number of dependencies: What realization of “sea level” is being referenced? Did you measure relative to a local reference mark with a published value? If so, how was that value established and to what realization of “sea level?” GNSS is in many ways the most practical method to use for such surveys, but it will only yield an ellipsoid height value. A precise geoid difference model needs to be applied to derive an orthometric elevation. But this leads to more questions: How well were the gravity measurements referenced to established geodetic frameworks in order to develop the geoid difference model?
The challenges do not end there. Even with a refined geoid model, using GNSS observations alone (e.g., without real-time corrections or post-processing base station data) would not yield high precision. With post-processing methods, the positions of the bases need to have a geodetic connection to the same reference framework as the geoid difference model. These interwoven geodetic dependencies do not make for a “quick survey” solution. Fully recognizing these realities, the Survey Department of Nepal embarked on one of the most comprehensive and high-tech geodetic surveys ever undertaken in the region.
“We established a network of 298 survey marks around the area of the mountain and did gravity measurements and GNSS on each,” said Susheel Dangol, the coordinator and Chief Survey Officer of the Sagarmatha Height Measurement Secretariat. “[Trimble] DiNi digital levels and trigonometric leveling with total stations were used to establish precise elevations on a subset of these marks to serve as primary points. And we did GNSS observations on each for horizontal positions.”
To provide base data for the GNSS observations on the reference network and subsequently the survey of the peak, eight temporary CORS were setup on and around the mountain. Mostly these CORS were set up at elevations between 5,000 to 6,000 meters — several by helicopter. This network was constrained by long static sessions and adjusting to a national CORS network. Nepal’s CORS network was built in cooperation with the international scientific community to study the plate tectonics of the region; this large-scale geodetic undertaking was phased over multiple years and is estimated to have a final cost of nearly $1.3 million (USD).
As a national mapping agency, the Survey Department of Nepal does extensive geodetic, engineering, and cadastral surveying. To meet these surveying needs, they have a large existing stock of modern instruments. However, according to Dangol, to ensure they had the latest gear with the most capabilities and well suited for the harsh environments for the project, surveying instrument manufacturers contributed to the project.
“Trimble India brought us one Net R9 base receiver, two R10 [rovers] for the survey of the peak, and three S9 total stations that we used to measure our baselines for the reference network, and a license of TBC (Trimble Business Center Software) for our GNSS post-processing,” said Dangol. “A team from Trimble New Zealand also came to help set us up with the new instruments.” Trimble receivers have been on the summit on numerous occasions including in the 1998 American Mount Everest Expedition; the 1999 U.S. National Geographic Society expedition; as well as others in 2005, 2013 and 2015.
Also watch: Nepal Survey Department Uses Trimble Tech to Measure Mount Everest
The window
With the reference framework established and the CORS collecting data for post processing, the team was ready to take on the peak. The climbers were well acclimated, having done training climbs at high altitudes, and for Gautam this would be his second trek to the summit — but his first with surveying tasks added. In addition to climbing gear, oxygen supplies and over 41Kg (90 lbs.) of surveying gear needed to be relayed through successive camps to the South Col — the final camp before summit attempt.
To try to ensure optimal conditions on the summit for the GPS and GPR measurements, the team chose the month of May. And while no conditions on this unforgiving mountain could be fairly characterized as favorable, the April-May window was the best bet. Unfortunately, April-May is also the peak of the climbing season, and this can result in actual “traffic jams” at key points along the most popular routes, like at the narrow Hillary Step just below the summit.
The surveyors chose the same route that Sir Edmund Hillary and Tenzing Norgay took on their historic ascent in 1953: The South Col route, which is still the most popular. The team needed to negotiate the treacherous Khumbu Icefall and Western Cwm, then up the Lhotse Face to the South Col where they set up camp before the final push to the summit. This is a popular spot for a final camp and a staging point for equipment and oxygen supplies relayed up the mountain by Sherpas. It is also one of the most perilous places on earth.
The South Col is the beginning of what is called the death zone, where the risks come from not only the harsh wind and cold — at that altitude, certain bodily functions are compromised. Supplemental oxygen is viewed as an imperative for climbers, sleep is near impossible, and digestion all but shuts down — the body will often feed off reserves rather than trying to digest food.
At the South Col camp, they tried to get some rest, which is difficult at that altitude for even the most experienced climbers. The two surveyors and three of their four Sherpas then set off for the summit much earlier than most climbing parties. This was to avoid crowds at the summit and provide time to perform the surveying tasks. Reaching the summit at 3:00 am, they stayed there for almost two hours, much longer than most who make the summit. Read the first-hand account from the summit leader, Khim Lal Gautam, in the companion article: Two Hours on the Summit.
One of the two Trimble R10’s was set up on a mini-tripod with a quick-release attachment. The R10 had been pre-configured to begin recording static observations on power-up. This was fortunate, as a data controller would have been impractical; removing heavy gloves or using gloves thin enough to operate a controller keyboard could be dangerous. As it was, lighter gloves were required to operate a data controller connected by a cable to the GPR receiver, which (unfortunately) contributed to the frostbite that some of the team experienced.
While the R10 collected the static session — which it did for 1 hour and 16 minutes — the surveyors performed the GPR observations needed to determine the highest point of solid ground below the ice and snow cap. “They did a pattern with the GPR, from east to west, and north to south,” said Dangol. “The area of the summit is about five to six meters from side to side. We did not have any difficulties with the GNSS or the GPR: the instruments were carried to the top in insulated bags and we did not have to charge any batteries. There was a fully charged battery in the R10 and we had spares.”
Dangol and the surveyors said they were pleased with the GNSS receiver, found it to be light and compact and had no issues during the climb and numerous training and test sessions. Reliability was key; there was no plan B at the peak. The R10 weighs just over 1 Kg (2.25 lbs.) and operates in temperatures from -40°C to +65°C (-40°F to +149°F), a good fit for Everest’s summit averages for May of -15°F and -26°C. But it was likely much colder during the 3:00a.m. survey.
There was little time for photos as the oxygen supplies were running low. So low in fact that during the descent, which can be the most perilous part of the journey, one of the team members ran out of oxygen and was in risk of death. An oxygen bottle borrowed from a different team of Sherpas was credited with saving his life. Gautam also suffered some permanent damage from frostbite. Otherwise, the team returned safely and with the desired data; the equipment had performed flawlessly.
The static data collected by the R10 on the summit and the eight CORS running simultaneously includes observations from four constellations: GPS, Glonass, Galileo and Beidou. The data were processed in TBC software at the main offices of the Survey Department of Nepal. Using multiple constellations ensures optimal geometry and is a plus for relatively short observations times. As it turned out, the 76 minutes of observations were more than adequate.
The announcement
In a joint virtual event on December 8, 2020, representatives from China and Nepal pressed buttons simultaneously and the new elevation appeared on the screen: 8,848.86m.
The announcement was long in coming as it had been over 18 months since the Nepal survey, and more than six months since the Chinese survey. Each team took time to carefully analyze their own data. Much of the delay was also likely due to the nature of the cooperative arrangement between these two countries.
Each team needed to maintain autonomy over their own surveys and compute them separately. There were similar elements: both deployed GNSS receivers and GPR to the summit, and both used networks of GNSS reference stations to constrain computations to. Beyond that, however, there were differences. Nepal’s survey was part of a wide region update of their geodetic reference framework. But not as much is known (publicly) about the broader elements of the Chinese survey. It is unlikely that the independent survey data was processed together, and computed as a single solution. But techniques would have been compared and discussions held on how to report results in a common reference. Coming to an agreement on an elevation value would have taken some time, but more time was added due to planning the details and messaging for the joint announcement.
The decision to jointly announce came from diplomatic outreach. In October 2019, President Xi of China visited Nepal and while there to discuss other matters, the subject of the elevation of Everest came up. China had been planning their own re-survey of the peak and the two countries agreed to wait and do a joint announcement of the results of both surveys. The Chinese survey team summited on May 27, 2020.
Dangol beams with satisfaction over the performance of the summit team and the entire team involved in the broader geodetic project. “There has been splendid coordination between all of our people,” said Dangol. “We have a very hard-working team, who had to survey in harsh conditions, not just the summit survey. Much of the fieldwork for the [broader regional] geodetic survey was also mostly at high altitudes and in inclement weather. The project was a matter of national pride for Nepal and a prestigious undertaking for the Nepali government. I feel very proud that we were able to complete it successfully. This is an amazing team.” It is part of the national identity of the Nepalese that they are truly the masters of their own mountains in so many ways. And now the distinction of mastering the highest altitude surveying on earth can be added to this source of national pride.
