Coordinate system
The surface of the earth is not a standard perfect sphere. According to the measurement results in 2020, the height of Mount Everest is 8848.86m, and the depth of the deepest trench on the earth - Mariana Trench is 11034 m. The difference between the two is nearly 20 km! Because the earth's natural surface is uneven and complex, it obviously cannot be used as a datum for measurement. Therefore, people began to seek a regular surface that is close to the natural surface of the earth to replace the irregular earth surface. The ocean area accounts for about 71% of the earth's surface area, and the land area only accounts for 29%. Therefore, the water level is used to represent the physical surface of the earth. Simply put, it is assumed that there is a static sea surface (a static sea surface with no waves, no tides, no currents, no changes in atmospheric pressure, and in a state of fluid equilibrium) extending toward the land. A closed surface representing the physical surface of the Earth.
In reality, the sea surface will continue to change due to the influence of tides, so people set up tide gauge stations on the seashore and calculate the average height of the sea surface through long-term observations. This level passing through the average height is called the geoid , also known as the elevation datum . The current elevation datum stipulated in our country is 985 elevation datum .
Reference: National Basic Technical Regulations on Geodetic Surveying GB 22021-2008
The geoid ignores the unevenness on the ground, but due to the uneven distribution of mass within the earth, the geoid is still undulating. Although it is very close to a regular ellipsoid, it is not completely regular and cannot be defined and expressed using a mathematical model.
So people further assumed that the surface of a rotating ellipsoid body is used to approximate the geoid, also known as the earth's ellipsoid. spheroid is a shape formed by rotating an ellipse around its minor axis. Its major semiaxis a (equatorial radius) is larger than the minor semiaxis b (polar radius). flat rate f=(a-b)/a.
In different historical periods, different countries and regions, ellipsoids of different sizes have been calculated based on corresponding data to approximately represent the geoid.
The list of earth ellipsoids used in our country in different periods is as follows:
With the ellipsoid, the coordinate system can be established based on the ellipsoid to express the position of a certain point on the ground. But there is still a problem at this time: when using an ellipsoid to "fit" different areas, some places will "fit" well and some places will not "fit" well, so there is still a problem. A geodetic datum is needed to determine the "position" and "attitude" of the ellipsoid.
The geodetic datum includes two categories:
The geocentric datum : obtained from the satellite data, using the earth's center of mass as the origin to best fit the global scale.
Regional datum : It coincides with the earth's surface in a specific area. The geodetic origin is the point where the reference ellipsoid is tangent to the geoid.
The difference between the geocentric coordinate system and the geocentric coordinate system:
The geocentric coordinate system : refers to that after positioning and orientation, the center of the earth's ellipsoid does not coincide with the earth's center of mass but is close to the earth's center of mass, which is regional (local scope) coordinate system.
geocentric coordinate system : refers to the global coordinate system in which the center of the earth's ellipsoid coincides with the earth's center of mass after positioning and orientation.
The above process from the natural surface of the earth - the geoid - the earth ellipsoid - the geodetic datum can be regarded as a "step-by-step approach" to the earth. With the development of surveying and mapping science and technology, the geodetic coordinate systems used in my country in different periods are as follows:
At present, my country has fully implemented the use of the 2000 national geodetic coordinate system [China Geodetic Coordinate System 2000] (ie CGCS2000 coordinate system).
So if the latitude and longitude coordinates of a certain place are [116.56, 39.97], it does not mean that the location described can be accurately marked on the map, because the data obtained from the same place collected in different coordinate systems is different. . Therefore, if you want to accurately mark the location of a certain place on the map, in addition to knowing the coordinate value, you also need to know the coordinate system in which the coordinate value was collected and produced.
In the field of GIS, coordinate systems are generally divided into geographic coordinate system (Geographic Coordinate Systems) and projected coordinate system (Projected Coordinate Systems). If you also want to define the elevation of a certain point (which can be simply understood as altitude), strictly speaking, you also need the elevation coordinate system , sometimes also called the vertical coordinate system (Vertical Coordinate Systems).
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