How is the latest height of Mount Everest measured
Jiang Tao
On December 8, China and Nepal jointly announced that after the solid work of the teams of the two countries, the latest elevation of Mount Everest is 88.886 meters.
The height measurement of Mount Everest this time is also the fourth large-scale survey and scientific expedition of Mount Everest.
How is the latest height of Mount Everest measured?
What kind of progress has been made in the technical means of human measurement of Mount Everest?
This newspaper specially invited Jiang Tao, a member of the 2020 Mount Everest Height Survey Technology Coordination Group and a member of the China-Nepal Mount Everest Survey Joint Technical Committee, to write an article to answer these questions to readers.
Mount Everest, referred to as Mount Everest, is the main peak of the Himalayas and the highest mountain in the world. It is located on the border between China and Nepal, with its north in Tingri County, Tibet, and Nepal in its south.
Tens of millions of years ago, the Indian Ocean plate collided and squeezed the Eurasian plate, and the Qinghai-Tibet Plateau gradually uplifted and formed the "Roof of the World". The representative mountain is Mount Everest.
As the world's highest peak, the exact height of Mount Everest has always been the focus of the world.
So, how is the height measurement of Mount Everest in 2020?
What is the scientific truth?
For the 2020 Mount Everest elevation survey, my country has adopted a variety of traditional methods such as “comprehensive use of GNSS satellite surveys, leveling surveys, photoelectric ranging, snow depth radar surveys, aerial gravity and remote sensing surveys, quasi-geoid refinement and real-world 3D modeling, etc. Modern surveying and mapping technology, the technical route of accurately measuring the height of Mount Everest, and solid technical cooperation with Nepal, finally determined that the snow elevation of Mount Everest based on the global elevation benchmark is 88.886 meters.
Specific scientific work can be divided into the following six levels.
Establish GNSS coordinate control network
A global navigation satellite system (GNSS) coordinate control network was established in the Mount Everest area. The survey team carried out high-precision GNSS network observations in stages and obtained the three-dimensional coordinates of 343 network points. This established a high-precision basis for the calculation of the coordinates of the Mount Everest elevation measurement.
Accurately transfer the yellow sea elevation reference value to the foot of Mount Everest
Deploy an elevation control network in Mount Everest and its surrounding areas to carry out leveling surveys, starting from the national first-class leveling point in the Xigaze region of Tibet, and surveying team members use precision levelers to accurately transfer the yellow sea elevation reference value to Mount Everest one station At the foot, a total of more than 780 kilometers of leveling surveys were completed, all of which required surveying team members to rely on walking surveys to complete.
First airborne gravity survey on the north side of Mount Everest
Carry out gravity surveys on Mount Everest and surrounding areas.
Accurate determination of the elevation of Mount Everest requires the establishment of a high-precision geoid model of the Mount Everest region, which in turn requires uniformly distributed high-precision gravity data.
General gravity measurement uses the method of ground gravity measurement. Surveying and mapping personnel carry a gravimeter for field measurement and place the instrument on a measurement point. After measuring for a period of time, the gravity data of this point can be obtained.
The average altitude of Mount Everest is more than 5,000 meters, and the topography is extremely complex. Most areas cannot carry out ground gravity measurement, gravity data is scarce, and there are a lot of blank areas of gravity data.
Therefore, for the first time in the world, we carried out airborne gravity surveys in the north side of Mount Everest to solve the problem of gravity data blanks and improve the accuracy of the elevation calculation surface of Mount Everest.
Airborne gravity measurement, as the name suggests, is to install an airborne gravimeter on an airplane, which is equivalent to installing a sensor that senses the earth's gravity on the airplane, which can reflect the changes in ground gravity.
The aircraft flies back and forth in the air according to the pre-designed survey lines, and multiple flying survey lines form a dense air gravity data surface.
We combined airborne satellite dynamic positioning, inertial navigation and gravimeter data to measure the air gravity value.
The aerial gravity survey on the northern side of Mount Everest used the Aerogeological No. 1 aircraft, equipped with an advanced aerial gravimeter, flying at an altitude of more than 10,000 meters, and obtained a total length of 5635.2 kilometers of aerial gravity survey lines, covering an area of 12,700 square kilometers.
The topography of Mount Everest itself is complex, and the data is incomplete. The plane takes off at high altitude and the wind is particularly strong. It can be said that it is a high-risk and difficult flight.
After repeatedly consulting with technical experts from the Air Force, Civil Aviation and Meteorological Departments, we made up our minds to carry out high-risk and difficult aerial surveys in the Everest region.
Survey climbers climb to the top to carry out peak surveys
The most important thing for the elevation measurement of Mount Everest is to measure the climbers climbed to the summit to carry out the summit survey and obtain the observation data of the summit.
At 11 o'clock on May 27, 2020, the Chinese Survey Mountaineering Team stayed on the summit for a record 150 minutes after successfully climbing.
At the top of the peak, measurement target erection, GNSS measurement, snow depth radar measurement and ground gravity measurement were carried out, all using domestic instruments.
The survey climbers achieved the Beidou satellite positioning on the summit for the first time, and the observation time exceeded 40 minutes. The summit and the 9 GNSS ground stations in the Everest region formed the summit GNSS joint survey network for simultaneous GNSS observation.
The surveying climbers used the domestically-made gravimeter to measure the gravity observations of the top of Mount Everest for the first time in the world, which helped to improve the accuracy of the starting surface of Mount Everest.
In addition, the thickness of the ice and snow layer on the top of the peak was measured with a domestic geological radar detection instrument.
At the same time, ground survey personnel used the self-developed long-range rangefinder to collimate the peak-top target reflecting prism from 6 stations at the foot of Mount Everest for rendezvous observation. The longest range was close to 19 kilometers. The rendezvous observation data It mainly provides independent verification for peak GNSS measurement data.
The successful completion of the measurement in an extremely cold and extremely low-pressure high-altitude environment such as the summit of Mount Everest is a good illustration of the great progress of domestic measuring instruments.
Everest measurement data processing analysis
After all the measurement data has been acquired and passed the quality inspection, it enters the Everest measurement data processing and analysis stage.
In a nutshell, the core of data processing for Mount Everest elevation measurement is to obtain the precise positions of the "top" and "bottom" of Mount Everest through rigorous calculations.
By processing the GNSS control network, the peak GNSS joint measurement network and the intersection measurement data, the three-dimensional spatial coordinates of the target point on the peak of Everest can be obtained, that is, the latitude, longitude and height of the earth.
The data processing results show that the difference between the geodetic height of Mount Everest calculated using GNSS measurement data and the result determined by the rendezvous survey is only 2.6 cm. Considering the independence of the two technical means, the difference in this magnitude is very small; according to the error theory, The accuracy of the result of the GNSS survey on Mount Everest reaches the millimeter level.
This realizes the high-precision determination of the "top" of Mount Everest.
Next, we need to accurately calculate the position of the "bottom" of Mount Everest. Scientists perform data processing based on the theoretical methods of physical geodetic surveys, combine aviation gravity, ground gravity, high-resolution topography and other data, and combine GNSS and leveling data to establish Mount Everest. The geoid model of the region is equivalent to accurately determining the "bottom" of the elevation base of Mount Everest, which is the base level of the Yellow Sea elevation.
The results show that after adding the airborne gravity measurement data, the accuracy of the height calculation surface of Mount Everest reaches 4.8 cm, which is nearly 40% higher than when there is no airborne gravity measurement data, which means that the accuracy of the altitude of Mount Everest will also be corresponding. Promote.
The "top" and "bottom" of Mount Everest have been accurately determined. By subtracting the geoid gap from the top of Mount Everest, the precise elevation value of Mount Everest can be obtained.
China and Nigeria cooperate in data processing
The overall goal of the 2020 Mount Everest elevation measurement is to implement the China-Nepal joint statement and realize the joint declaration of the Mount Everest elevation by China and Nepal.
Both China and Nepal established the China-Nepal Joint Technical Committee for Everest Measurement and cooperated in data processing.
Elevation datum is a key element of the height measurement of Mount Everest. China and Nepal have their own national statutory elevation datums. China is the average sea level of the Yellow Sea and Nepal is the average sea level of the Indian Ocean.
According to the practice of international cooperation, the common elevation datum is usually used in international cooperation, instead of the elevation datum of one party.
The China-Nepal Joint Technical Committee finally agreed through technical talks: According to the definition and parameters of the Global Elevation Reference (IHRS) issued by the International Geodetic Association, the ground gravity, aviation gravity and other data will be combined to establish the gravity geoid in the Mount Everest region. This is The concrete realization of the global elevation benchmark in the Mount Everest region serves as the starting point for China and Nepal to jointly announce the elevation of Mount Everest.
After selecting the elevation benchmark, during the data processing stage, China and Nigeria carried out detailed comparisons and repeated checks on the intermediate data and results one by one. The data and results of both parties are very consistent. Finally, the technical teams of the two sides jointly determined the global elevation benchmark. The snow elevation of Mount Everest is 88.886 meters.
In addition, in the 2020 Mount Everest elevation measurement work, my country has also carried out aerial remote sensing surveys, real-world 3D model construction and glacier change monitoring in the Mount Everest region.
The rich observation data obtained by the 2020 Everest survey will provide valuable first-hand observation data for the ecological environment protection restoration, natural resource management, geological research and survey in the Everest region.
Further reading
Historical review of the height of Mount Everest
Humans’ understanding of Mount Everest began by measuring its height.
In history, the technology of height measurement of Mount Everest has also undergone continuous development.
Triangular elevation measurement without personnel climbing
From 1714 to 1715, under the dispatch of Emperor Kangxi of the Qing Dynasty, the surveying personnel of the Qing Dynasty went deep into the foot of Mount Everest, and used the copper gilded full circle meter, the copper imperial square square quadrant (now in the Palace Museum) and other instruments to measure the pearl. The location and height of the peak have been initially measured, and the location and name of Mount Everest have been clearly marked on the "Huangyu General Map".
This is the first time humans have surveyed and mapped Mount Everest.
In 1847, the British Indian Survey Bureau used the triangular elevation measurement method to measure the height of Mount Everest for the first time on the Ganges Plain, more than 300 kilometers away from Mount Everest, and the measured elevation was 8778 meters.
The so-called triangular elevation measurement method is to set up an optical measuring instrument at the ground station at the foot of the mountain. Professional surveyors aim at the snow surface of Mount Everest, measure the distance from the ground station to the peak, and then measure the height angle of the peak. It is to observe the angle between the line of sight and the horizontal plane, and finally calculate the height of Mount Everest according to the trigonometric function relationship.
In the period when humans have not yet reached the summit of Mount Everest, the triangular elevation measurement of the height of Mount Everest is a very ingenious remote measurement technology.
However, because of the long distance from the ground station to the top of the peak, even a small height angle measurement error will cause a large height error.
In addition, the atmosphere is a sparse upper and lower dense circle layer, and light will be refracted when passing through media of different density.
When looking at the top of the peak from a ground station, the line of sight passes through the atmosphere with a thickness of about 3,000 meters, and the reflected light will be significantly refraction, resulting in atmospheric refractive difference, leading to measurement errors in the height of Mount Everest.
From 1921 to 1954, the British Bureau of Survey of India carried out many measurements after improving its measuring instruments, techniques and methods. Through weighted calculations, a more accurate Mount Everest elevation of 8847.6 meters, approximately 8848 meters, was obtained.
Personnel climbed to the summit of Mount Everest and set up a measurement target
Sighting the snow surface of Mount Everest from the ground station, the targets observed by different stations are inconsistent, which will cause large errors. This is the defect of the triangular elevation measurement method.
In 1975, China carried out a large-scale survey of Mount Everest. For the first time, mountaineers set up a survey target at the highest point of the summit, and observed the target at the same time from 10 ground stations, ensuring that all ground stations were aiming at the peak. The highest point, and it's the same point.
Through the efforts of Chinese scientists, improved theoretical methods, combined with meteorological parameter observation data, the problem of atmospheric refractive error has been solved.
This time, a manual plunger was used to measure the thickness of the snow at the top of the peak to be 0.92 meters (excluding the icy layer).
In addition, the triangulation surveys, leveling surveys and gravity surveys carried out in the Mount Everest area, and related data provide the starting basis for the elevation of Mount Everest.
After obtaining all the measurement data, the elevation of Mount Everest is finally calculated as 8848.13 meters.
Satellite precision positioning technology to measure the height of Mount Everest
Since 1987, with the successful application of the Global Navigation Satellite System (GNSS), the height measurement of Mount Everest has entered the era of precision satellite positioning. Many countries in China and abroad have used GNSS technology to measure the height of Mount Everest.
GNSS measurement is the use of artificial satellites to accurately measure the position of the ground surface, the ground GNSS observation equipment is used to receive satellite signals, and the distance between the satellite and the ground measurement point is measured according to the signal propagation speed and time.
Since the distance from the satellite to the center of the earth can be known by precisely determining the satellite orbit, then the distance from the ground survey point to the earth's reference ellipsoid can be obtained, and this distance is called "Earth Height".
It should be pointed out that “high altitude” is not the “high altitude” that we often say. To obtain “high altitude”, one needs to determine the geoid, which means that it coincides with the average sea level and extends into the interior of the continent. The level surface that envelops the earth.
The geoid represents the average sea level, which is the starting surface or reference surface of the elevation. The altitude we usually call is the height value relative to this starting surface, which is why it is called the altitude.
Determining the geoid is to determine the distance between the geoid and the reference ellipsoid of the earth, that is, the geoid gap. This requires the use of densely distributed gravity data in the area and the determination based on the theory and methods of physical geodesy. Therefore, gravity is required. measuring.
The two large-scale Mount Everest surveys in my country in 1975 and 2005 used leveling and gravity measurement techniques to transfer the reference value of the elevation of the Yellow Sea in Qingdao to the Mount Everest area, realizing the determination of the starting surface of Mount Everest.
After accurately determining the height of the top of Mount Everest and the difference between the geoid, the former subtracts the latter to obtain the altitude of the top of Mount Everest, which is called orthometric height in surveying.
Thickness measurement of snow and ice layer on the top of Mount Everest
The top of Mount Everest is covered by snow and ice all year round. The method of measuring the depth of the ice and snow from the snow surface to the rock surface on the top of Mount Everest has been used until 2005.
In 2005, my country used a portable radar detector to accurately detect the thickness of the ice and snow layer on the summit of Mount Everest in 2005. The thickness of the ice and snow layer at the peak target point was 3.50 meters, and the measurement accuracy was ±0.1 meters.