Nepal, China re-measure height of Mt Everest
If it was not for the Covid-19 pandemic, we would by now have found out the result of the latest measurement of the world’s highest mountain.
Yes, 8,848m (29,028ft) is widely accepted as the height of Mt Everest since 1955, even though the elevation of the summit has fluctuated by a dozen or so metres since then, depending on who is calculating.
Since previous measurements were by American, European or Indian surveyors, it has now become a matter of national pride for Nepal and China to jointly come up with their own figure for the imposing peak that lies on their border.
The height of Mt Everest has changed with advances in survey technology. The Great Trigonometric Survey Of India in 1856 declared Mt Everest as the highest mountain in the world at exactly 29,000ft. But since surveyors thought no one would believe such a round figure, they added 2ft to it so it sounded more precise.
There are also many different ways to calculate the elevation of Mt Everest: from mean sea level, from the lowest depth of the ocean, or even the distance from the centre of the Earth which would mean that some Andean peaks would actually be ‘taller’ because the planet is not perfectly round but has a slight bulge along its equator.
Then there is the question of whether the true elevation is the bedrock at the summit, or should the 4m thick icing on the top of Everest also be added to the height? The height is also not static – the peak is undergoing tectonic uplift of about 1cm a year, and the ice cover on the summit varies in thickness because of precipitation, wind and (of late) global warming.
More recently, geologists and scientists have also come up with different figures for the impact of the 2015 earthquake– most agree that Mt Everest may have shrunk by about 3cm, and been displaced southwards by a metre or more.
“The height of Everest is constantly changing due to tectonic activity, of which the 2015 Gorkha Earthquake is only the most recent contribution,” explains Christopher Pearson of the University of Otago in New Zealand who worked with Nepal surveyors on the measurement in 2018. “Since the present measurements are fairly old, we needed to update it.”
The New Zealand government assisted Nepal’s Survey Department to install GPS equipment on the summit and train technicians in processing the GPS data, geodesy, levelling and gravity measurements.
Says Navin Manandhar, formerly with the Geodetic Branch of the Survey Department who helped develop the methodology for re-measurement: “This body of work is a matter of pride for Nepal, but is also a huge undertaking and with the whole world keenly waiting for the result, it has not been easy.”
Indeed, Survey Department officials are tight-lipped about the final height, and will not even give a hint about whether Mt Everest is taller or shorter. The reason for the sensitivity is that the governments of China and Nepal have signed an MoU to cooperate in the re-measurement and agreed during President Xi Jinping’s state visit to Kathmandu last October to announce the result jointly.
Even if the new height of Mt Everest is found to be lower than it is now, there is no danger of its demotion from being the highest mountain in the world. The second highest peak, Mr K2 in Pakistan is only 8,611m.
Nepal’s own survey team led by Khim Lal Gautam climbed the mountain on 22 May 2019, and installed GPS equipment and ground-penetrating radar at the summit. The team risked its life, with Gautam losing a toe to frostbite in the process.
The Chinese side has not completed its study yet. A Chinese survey team finally climbed Mt Everest from the north in the midst of the coronavirus pandemicon 28 May, and left its own equipment on top. This allowed measurements with China’s BeiDou satellite navigation system, as well as a series of overflights above Mt Everest in June by an aircraft installed with precision gravity survey equipment.
There is now concern about what happens if there is discrepancy between the Chinese and Nepali calculations of the height. But experts interviewed for this report said the two elevations would be “close” even though China used the Yellow Sea as a reference for sea-level, whereas Nepal has chosen a point much closer on the Bay of Bengal coast.
Says Pearson: “I expect the program will come up with by far the most accurate sea level height for Sagarmatha since Nepal has used cutting edge features in its survey. This work has also led to better models of geoid systems which will make it easier to measure the exact heights of other mountains in Nepal using modern GPS techniques.”
Nepal’s Survey Department deployed both traditional methods (trigonometry levelling as done by Survey of India in 1950s) with precise levelling and more accurate modern geoid system with gravity survey, and Global Navigation Satellite System (GNSS) to cross check the results.
Under trigonometry levelling, the Survey Department selected 12 observation points in Solukhumbu, Khotang, Bhojpur and Dolakha districts from where its technicians measured the position and angle of the summit. Surveyors then used Pythagorean theorem to measure its height.
For precise levelling, the surveyors had to measure altitudes of a series of 12 points from sea level right up to the base of the mountain. For this, surveyors had to use six Indian trigonometrical points from the sea to the Nepal border.
The ground penetrating radar that the Survey Department team left on the summit of Mt Everest last May measures the thickness of the ice cap at the summit and whether it is thicker or thinner. This can be used to calculate the rock height of the summit.
Nepal’s surveyors have completed their calculations, and say that almost as challenging now is to keep that number a secret until the Chinese complete their calculations, and a joint announcement is made. That date has been pushed back by the pandemic.
Even though the two countries share the mountain with the north, west and east face in China and the south face in Nepal, Mt Everest (Sagarmatha or Chomolungma) is an emotive issue in the country.
“It will be unfortunate if this purely scientific exercise is overshadowed by politics,” says Manandhar. “It will help bilateral relations, but this exercise goes beyond Everest. The knowledge and experience we have gained will be crucial in all future survey work in Nepal.”
How high is high?
The modern history of Mt Everest is replete with debate about its true height. The Great Trigonometric Survey (GTS) of India in 1856 declared the mountain the highest in the world at 8,840m – and named the peak after Sir George Everest, its chief in 1965.
In 1955, the Survey of India revised the height as 8,848m, and this was reconfirmed by China (8,848.13m) in 1975. But in 1999, a US survey using GPS and radar technology declared its height to be 8,850m.
In 2005, a Chinese expedition remeasured Everest’s rock height and claimed it to be four meters lower at 8,844.43m, giving rise to long-standing debate on what should be the official height. China and Nepal then agreed they would use both figures.
The problem with taking the ice cap as the summit height is that its thickness varies from year to year, whereas the rock summit is more permanent.