What are the stages of development of geodesy? What are the major contributions and landmark achievements of each stage?

Before the 17th century, geodesy was in its infancy. In the 3rd century B.C., Eratosthenes first applied the relationship between the length of a segment of an arc on the circumference of a circle, the corresponding central angle and the radius of the circle in geometry to calculate the length of the radius of the earth. In 724 A.D., Nangong said and others in the Tang Dynasty of China, under the guidance of Zhang Sui (one line), first measured a meridian arc of about 300 kilometers in length in present-day Henan Province. Similar work was carried out in other countries. However, at that time, the measuring tools were simple, the technology was rough, and the results obtained were not very accurate, just an attempt to measure the size of the earth.

Formation of geodesy

After I. Newton published the law of gravity in 1687, C. Huygens of Holland published "On the Causes of Gravity" in 1690, according to the law of gravity on the surface of the earth from the equator to the poles, he concluded that the earth's shape is an oblate spheroid with slightly flattened poles, and A.C. Croix de Chatelot of France published "Theory of Earth Shape" in 1743, which further gave a theory of the shape of the earth from gravity to the poles. In 1743, A.-C. Clerot of France published "Theory of Earth Shape", which further gave Clerot's theorem of determining the flatness of the earth from gravity data and the angular velocity of earth's rotation. In addition, at the beginning of the 17th century, W. Snell of the Netherlands pioneered triangulation. Subsequently, the invention of telescope, micrometer, level, etc., the accuracy of measuring instruments greatly improved, laying the technical foundation for the development of geodesy. 17th century, the formation of geodesy to the emergence of satellite geodesy, this stage of geodesy is usually called classical geodesy. The main symbol is the ground angle measurement, ranging, leveling and gravity measurement as a technical means to solve the problem of land regional geodesy. Arc measurement, triangulation, geometric elevation measurement and the development of ellipsoidal geodetic theory, the formation of geometric geodesy; the establishment of the gravity field of the bit theory and the development of the ground gravity measurement, the formation of physical geodesy.

Arc Measurement

In 1683-1718, the French Cassini father and son (G.D. Cassini and J. Cassini) in the meridian circle through Paris with triangulation method to measure the arc length of the arc amplitude up to 8 ° 20 'arc length, the deduction of the earth's ellipsoid long semiaxis and flatness. The astronomical latitude observations were not made with the necessary precision, and the proximity of the two arc segments was such that a negative value of oblateness was obtained, i.e., the shape of the earth was that of an ellipsoid with two elongated poles, contrary to Huygens' deduction based on the laws of mechanics. In order to solve this doubt, the French Academy of Sciences in 1735 sent two surveying teams to the high latitude region of Lapland (located in Sweden and Finland on the border) and near the equator of the region of Peru to conduct meridian arc measurements, all the work was completed in 1744. The results of the two measurements confirmed that the higher the latitude, the longer the meridian arc per degree, i.e., the shape of the Earth is an ellipsoid with slightly flattened poles. Thus, the physics of the shape of the Earth was strongly supported by the results of the arc measurements.

Another famous radian measurement was that of the French meridian arc of 9°40', carried out by J.B.J. Delambre between 1792 and 1798. From the data of this new meridian arc and the Peruvian meridian arc measured between 1735 and 1744, the length of the arc in one quadrant of the meridian circle was deduced, and one ten-millionth of it was taken as the unit of length and named one meter. This was the origin of the meter system.

From the 18th century, after France, some European countries have also carried out the work of arc measurement, and the deployment of the way from the direction along the meridian to the development of vertical and horizontal intersection of the triangular lock or triangular network. This kind of work is no longer called arc measurement, but is called astronomical geodesy. A large-scale astronomical geodetic survey was carried out during the Kangxi period (1708-1718) of the Qing Dynasty in China for the purpose of compiling the "Emperor's Opinion of the whole map". In this survey, it was also confirmed that the meridian arc per degree at high latitudes was longer than that at low latitudes. In addition, the Kangxi Emperor of the Qing Dynasty decided to determine the length of the mile by using 200 miles per degree of meridian arc.

Geometric Geodesy

Since the 19th century, many countries have carried out national astronomical geodesy, the purpose of which is not only to determine the size of the Earth's ellipsoid, but also to provide precise geometric positions of a large number of ground points for the preparation of national topographic maps. This promoted the development of geometric geodesy.

1) In order to check and calibrate a large amount of observation data of astronomical geodesy, to find out the most reliable result and to evaluate the observation accuracy, A.-M. Legendre of France published the theory of least squares for the first time in 1806. In fact, the German mathematician and geodesist C.F. Gauss had already applied this theory to project the orbit of asteroid in 1794, and since then, he had used the method of least squares to deal with the results of astronomical geodesy and developed it to a fairly perfect degree, forming the method of leveling of measurements, which is still widely used in geodesy.

②The solution of triangles on the ellipsoid and the derivation of geodetic coordinates, Gauss proposed the solution of triangles on the ellipsoid in his work "General Theory of Surfaces" in 1828. Regarding the derivation of geodesic coordinates, many scholars have proposed various formulas, and Gauss published the orthographic projection method of projecting the ellipsoid onto the plane in 1822, which is the best method of converting geodesic coordinates into plane coordinates, and is still widely used today.

③The use of astronomical geodesy results to project the long semi-axis of the earth's ellipsoid and oblateness, the German F.R. Helmut proposed in the astronomical geodesic network of all astronomical points in the plumb line deviation of the sum of the square of the smallest conditions, the solution and the regional geodetic level of the best-fit ellipsoid parameters and their location in the earth's body method. Later this method was known as the area method.

Physical Geodesy

The most important development in physical geodesy since Clerot's publication of The Theory of the Shape of the Earth in 1743 was the Stokes Theorem, formulated by G. G. Stokes of England in 1849. According to this theorem, the shape of the geoid can be studied using ground gravity measurements. However, it requires that the results of ground gravity measurements be firstly imputed to the geoid, which cannot be strictly realized because the density of the earth's crust is unknown. Nevertheless, Stokes' theorem gave impetus to the study of the shape of the geoid. About 100 years later, M.S. Molodjinsky of the Soviet Union proposed the Molodjinsky theory in 1945, which can strictly determine the distance from the ground point to the reference ellipsoid (geoid) directly using ground gravity measurements without any normalization. It avoids the geoid, which cannot be strictly determined theoretically, and directly determines the geodetic elevation of ground points. Using this elevation, the ground observation values of geodesy can be accurately imputed to the ellipsoid, so that the processing of the results of astronomical geodesy does not bring errors due to inaccurate imputation. The astronomical gravity level measurement method and normal altitude system that accompanied Molodjensky's theory have been adopted by many countries. This is the theory and method of studying the shape of the earth and determining the earth's gravity field by ground gravity measurements, called classical physical geodesy, before the advent of satellite gravity measurement technology.

Modern geodesy

Classical geodesy due to its main measurement technology (measurement of angles and edges) and the limitations of the method itself, the accuracy of the measurement has been close to the limit, and the measurement range is difficult to reach 70% of the earth's area of the ocean and land areas with harsh natural conditions (plateaus, deserts and primeval forests, etc.). 1957, after the launch of the first Artificial Earth Satellite (AES), the use of artificial satellites to conduct geodesy has become a major challenge. After the successful launch of the first artificial Earth satellite in 1957, the use of artificial satellites for geodesy became the main technical means, and has since developed into modern geodesy. Its symbol is the generation of satellite geodesy, breaking through the meter-level measurement accuracy, from the regional relative geodesy to the development of global geodesy, from the measurement of static earth development to the measurement of the dynamics of the earth's effect.

Satellite geodesy

In 1966, the United States of W.M. Cowra published the book "Satellite Geodesy Theory", laying the foundation for the development of satellite geodesy. At the same time, the satellite tracking observation orbiting technology has been rapid development, from photographic observation to satellite laser ranging (8LR) and satellite Doppler observation. 1970s the United States first established satellite Doppler navigation and positioning system, according to the precise determination of the satellite orbit root number, able to earth 1 meter or higher precision determination of any ground point in the global geodetic coordinate system in the geocentric coordinates; 90s the United States and the development of a new generation of navigation and positioning systems, namely In the 1990s, the U.S. developed a new generation of navigation and positioning system, namely, the Global Positioning System (GPS), which is rapidly popularized globally with its advantages of cheapness, convenience and all-weather conditions, and has become a conventional technology for geodetic positioning. Russia has developed the Global Navigation Satellite System (GLONASS), and Europe is launching the Galileo global satellite navigation and positioning system (Galileo). Satellite geodesy is widely used not only for high-precision determination of the position of ground points, but also for determining the global gravity field and forming a new branch of geodesy, satellite gravity.

Satellite Gravity Measurement

Satellite laser ranging satellite tracking measurements can accurately determine the satellite orbit uptake, when the separation of the main part of the uptake of the Earth's gravitational uptake, which deduced the Earth's gravitational level of the sphere of harmonic expansion of the low-order coefficients of the Earth's gravitational level. 1970s satellite radar altimetry, and then the development of a number of generations of satellite altimetry system for accurate determination of average Sea surface geodetic height, determine the ocean geoid, and inverse ocean gravity anomaly, the resolution is better than lO kilometers, the accuracy is better than the decimeter level.

Dynamic geodesy

SLR and very long baseline interferometry (VLBI) can monitor the rate of plate motion, pole shift and changes in the rate of rotation of the Earth with centimeter-level or better accuracy, and GPS can determine the relative motion of the plate and plate land masses and crustal deformation with millimeter-level accuracy, and it is also widely used to monitor the faults and seismicity, the motion and change of the polar ice sheets and land glaciers, and the phenomenon of post-ice rebound. It is also widely used to monitor faults and seismic activity, the movement and changes of polar ice sheets and land glaciers, and the phenomenon of ice rebound.

Marine geodesy

Satellite altimetry has become the cheapest and most effective means of determining the high-resolution global oceanic geoid, and GPS has become the main tool for marine navigation and positioning, with a positioning accuracy of 1 to 2 orders of magnitude higher than that of the traditional astronautical and radio navigation, and the relative accuracy of multibeam sonar sounding has reached or is close to 111,000. The scale and accuracy of the undersea geodetic control network and seabed topographic surveys are constantly improving. [2]