Determining the size of the earth in geodesy refers to determining the size of the earth ellipsoid; Studying the shape of the earth is studying the shape of the geoid; Determining the geometric position of the ground point refers to determining the position of the ground point with reference to the ellipsoid of the earth. The ground point is projected onto the ellipsoid along the normal direction. The horizontal position of the point is expressed by the geodetic latitude and longitude of the projected point on the ellipsoid, and the geodetic elevation of the point is expressed by the normal distance from the ground point to the projected point. The geometric position of this point can also be expressed by three-dimensional coordinates in the spatial rectangular coordinate system with the center of mass of the earth as the origin. Geodetic work provides plane control network and elevation control network for large-scale topographic mapping; Provide control basis for economic construction such as developing mines, building water conservancy and developing transportation; Provide accurate ground point coordinates and gravity field data for launching missiles and spacecraft; It provides measurement data for the research tasks of geophysics, geodynamics and seismology. A brief history of geodesy has a long history. In the 3rd century BC, Eratosthenes of Alexandria first calculated the circumference of meridian circle by observing the sun shadows in two places, which was the original form of arc measurement. In 724, under the guidance of Zhang Sui (delegation), Nangong Shuo and others in China in the Tang Dynasty measured a meridian arc length of about 300 kilometers in present-day Henan, and measured the length of the sun shadow at the north and south points at the same time, and calculated the meridian arc length of latitude 1. This is the first time in the world to actually measure an arc. Other countries have done similar work in succession. /kloc-before the 0/7th century, the accuracy of the results was not high because of the simple tools and low technical level. 16 17, W. Snell of the Netherlands pioneered triangulation, which overcame the difficulty of measuring distance directly. Then came the invention of telescope, level meter and micrometer. The manufacturing of measuring instruments is gradually improved and the accuracy is improved, which lays a technical foundation for the development of geodesy. /kloc-at the end of 0/7, Englishman I Newton and Dutchman C Huygens studied the shape of the earth from the mechanical point of view, and put forward that the earth is an ellipsoid with slightly flat poles. From 1735 to 174 1, the French Academy of Sciences sent two survey teams to conduct arc surveys in Peru near the equator and Lapland near the Arctic Circle, respectively, and confirmed that the earth is an ellipsoid with slightly flattened poles. During the reign of Emperor Kangxi in Qing Dynasty, China carried out large-scale astronomical geodesy to compile the map of the empire. This survey found that the meridian arc per degree in the northeast of high latitude is longer than that in Hebei of low latitude and earlier than that in France. 1730, Xisong invented the theodolite, which promoted the development of triangulation. 1743, Clairaux published the theory of the shape of the earth, pointing out the method of accurately determining the oblateness of the earth through gravity measurement. 1806 A.-M. Legendre of France and C. F. Gauss of Germany published the least square theory respectively, which resulted in the survey adjustment method. 1849, Sir G.G. Stokes of Britain founded the theory of studying the shape of the horizon with gravity measurement results. 1880, Jedlin, Sweden put forward the method of measuring catenary baseline ruler, and then made Inva baseline ruler in France, which obviously improved the accuracy of ranging. At the end of 19 and 1930s, pendulum and gravimeter appeared successively, which greatly increased the number of gravity points and provided a lot of gravity data for studying the shape of the earth and its gravity field. In 1945, M.C. Molodenski of the Soviet Union proposed that the height of the earth from the ground point to the reference ellipsoid can be determined strictly, and the shape of the earth's surface can be determined directly without any reduction. This theory has been adopted by many countries. In the 1940s, the invention of electromagnetic wave rangefinder overcame the difficulty of ranging, and made traverse survey and trilateration get attention and development. 1957 After the successful launch of the first artificial earth satellite, satellite geodesy came into being, which made geodesy develop to a new stage. Navigation satellite Doppler positioning technology can determine the geocentric coordinates of any ground point in global geodetic coordinates, with an accuracy of 1 m or higher. Satellite radar altimetry technology can measure the fluctuation of ocean geoid. The newly developed satellite radio interferometry technology can measure the baseline component of the baseline vector between two points on the ground, that is, the three coordinate differences between the two points, in the three-axis direction of the global coordinate system. Satellite geodesy is still developing and has great potential. Branch geodesy includes geometric geodesy, physical geodesy, satellite geodesy, marine geodesy and dynamic geodesy. Geometric geodesy uses a rotating ellipsoid which is closest to the shape of the earth to represent the shape of the earth, determines its shape and size by geometric methods, studies and determines the geoid with this ellipsoid as a reference, and establishes a geodetic coordinate system to calculate the geometric position of ground points. Physical geodesy uses a gravity equipotential surface equal to the global average sea surface potential energy, that is, the geoid to represent the actual shape of the earth, carries out gravity measurement on the earth surface, and studies the fluctuation of the geoid relative to the earth ellipsoid with the ground gravity measurement data. Satellite geodesy uses the orbital motion of satellites in the earth's gravity field to observe the direction, distance or distance difference of the instantaneous position of satellites from dozens to dozens of tracking stations distributed as evenly as possible on the whole earth's surface, and accumulate long-term (several years) observation data of satellites with different heights and inclinations, so as to comprehensively calculate the geometric and physical parameters of the earth and the geometric positions of ground tracking stations relative to the center of mass of the earth.

Please apply for the measurement basis under classification.

/