Today, our national survey team once again reached the summit of Mount Everest, once again measuring the height of Mount Everest. This is the third time that the height of Mount Everest has been measured in China after 1975 and 2005. At present, the "height" of Mount Everest is still in the calculus stage, and we still need to wait for a while to know the new "height" of Mount Everest.
From 1975 to the present, the reason why we have to re-measure Everest every few years is because Everest is at the junction of the Eurasian plate and the South Asian subcontinent plate, and the plate movement makes the height of Everest change all the time, so it is necessary to update the height of Everest.
What to measure Everest?
In 1975, the Chinese surveying and mapping team adopted traditional geodetic surveying methods and erected the bid (a surveying tool) at the summit of Mount Everest. This is the first time in human history that the label has been brought to the summit of Mount Everest. At that time, the altitude of Mount Everest was accurately measured at 8848.13 meters, which was officially released to the public and recognized by the international community.
In 1975, the Chinese surveyors reached the summit of Mount Everest for the first time, and the red tripod was the Chan standard 丨 The Sports Bureau of the Tibet Autonomous Region
The traditional geodetic method mentioned here refers to the level measurement.
Schematic diagram of level measurement 丨 Author drawing
From the above figure, assuming that the height of point A is known as X, it is now necessary to calculate the height of point B. The steps to solve the problem are as follows:
1, set up a level measuring instrument between two points, read the readings of the level of A and B at two points respectively, Ha and Hb;
2, the difference between Ha and Hb H is the height difference between point A and point B;
3, the height of point B is X + H.
If you still need to calculate the height of point C, you can use X + H of point B as the starting point, move the level between point C and point B, and repeat the previous step.
This is the basic principle of traditional level measurement, which calculates the height of an unknown point from the height of a known point. When the height of Mount Everest was calculated, the Chinese survey team members selected the reference point at the foot of Mount Everest as the starting point, and moved the bid to the top of Mount Everest step by step.
"Altitude" is not simple
A necessary condition for traditional measurement methods is that it must start from a point of known height. As the first reference point for calculating altitude, the more accurate its height, the more accurate the value calculated later.
The altitude of a certain place that we usually say, many people think that the height difference between the ground and the sea level of this place is actually inaccurate, because the sea surface is affected by the tides, and the altitude changes all the time, so isn't the altitude of all the regions constantly changing?
To solve this problem, surveyors proposed that ideally, the height at sea level at rest would be used as the starting surface of altitude, and then this surface would be extended from the ocean below the land, forming a closed sphere covering the whole world, called the geoid. Because when the seawater is stationary, it is equivalent to the seawater being only affected by the Earth's gravitational field, so this geoid is also called the Earth's gravitational plane.
In China, the average sea surface of the Yellow Sea calculated by long-term observation of the Qingdao Port Tide Inspection Station is used as the horizontal base surface of China, and the so-called altitude of a certain place is the distance from the ground to this horizontal base. China's leveling origin is built near the Tide Inspection Station in Qingdao Port, and the height of the whole country is calculated from this point, which is equivalent to the A point in the above figure.
Photo by Yang Guang, Xinhua News Agency
However, due to the uneven distribution of the Earth's mass, the geoid is a sphere with a potholed surface, and it is not easy to accurately measure the local altitude somewhere. Therefore, people have come up with a way to calculate a mathematical sphere similar to the geoid through mathematical models, which is called a geoid-like plane. This mathematical sphere can calculate the surface position through latitude and longitude coordinates, which provides convenience for elevation calculation, and this is reasonable to be established by the mathematical model, so the gap between it and the geoid may be only a few millimeters. The elevation, altitude and the like that we are measuring now are actually using a geoid-like surface as the starting base surface, rather than the real geoid.
However, because each country is located in the earth's gravity field, countries will also use different geoid-like surfaces to meet their own needs. For example, India and we use different geoid-like models, so the same measurement of Mount Everest, the data released by our two countries are also different.
A new means of measuring the heights of Mount Everest
Traditional measurement methods have a second necessary condition: both front and rear points must be visible to the level at the same time.
Everest is covered in ice and snow all year round, and people can't observe the location of the top of the capped rock from a distance, so to measure the true height of Everest, it is necessary to exclude the influence of ice and snow - after all, the height of the mountain is the height of the top of the rock, not the height of the ice and snow at the top of the mountain. Humans have also climbed Mount Everest many times before, and the measurement method taken is to use a probe rod to probe the location of the rock layer of Mount Everest by using a probe rod to find a large hole under the ice.
Ice-covered Mount Everest 丨 Figureworm Creative
Now, with the advancement of technology, we have also adopted many new technologies to carry out this Everest measurement.
The first is that today's surveyors can use ground-penetrating radar to determine the highest point of Everest Rock and the thickness of the ice and snow above it, eliminating the need to drill holes as before. In addition, the height of Mount Everest was measured by satellite measurement - the team members installed a GNSS (Global Navigation Satellite Systems) antenna on the top of the target, placed it in a measured position for static observation, and measured the true height of Mount Everest.
The principle of satellite positioning is that the satellite broadcasts its own coordinates to the ground, and the receiver on the ground calculates the coordinates and elevation of the receiver's location according to the coordinates of the satellite after receiving the satellite signal.
Achieving satellite positioning requires at least four satellites. The satellite and the receiver form multiple triangles, each triangle of each side length, the degree of each angle of intersection is known, then we can use the trigonometric function to calculate the position and height of the receiver.
GNSS positioning principle 丨 Author drawing
There are currently four GNSS systems that provide positioning and elevation solutions, namely the Beidou system in China, the GPS system in the United States, the Galileo services system in Europe, and the GLONASS system in Russia. 15 years ago, we mainly relied on GPS signals to measure Mount Everest, and this time it was mainly Based on China's Beidou system, which also marked that the Beidou system has developed to a mature and reliable degree.