1 Introduction
Now our country has stopped using the Beijing 54 and Xi'an 80 coordinate systems, and switched to the CGCS2000 coordinate system. Beijing 54 and Xi'an 80 are the ginseng coordinate system, and CGCS2000 is the geocentric coordinate system which is the same as WGS84, but the ellipsoid oblateness is slightly different, and the actual coordinate results are almost the same, with a difference of about millimeters . CGCS2000 and WGS84 belong to the ITRS coordinate system. For details on what is ITRS, please refer to the blog post: Mutual conversion between coordinate system ICRS and ITRS, time system and conversion . Although the Beijing 54 and Xi'an 80 coordinate systems have been discontinued, they are still explained here, after all, it represents a large amount of history of the land in China.
2. Ellipsoid ginseng and earth barycentric coordinate system
2.1. Paracentric coordinate system
It is a geodetic coordinate system whose origin is the geometric center of the reference ellipsoid. "Centre" means the center of the reference ellipsoid. It is usually divided into: ginseng space Cartesian coordinate system (with X, Y, Z as its coordinate elements) and ginseng earth coordinate system (with B, L, H as its coordinate elements). The ginseng coordinate system is an O-XYZ coordinate system established within the reference ellipsoid. The origin O is the geometric center of the reference ellipsoid, the X-axis coincides with the intersection of the equatorial plane and the Greenwich meridian plane, and the eastward direction is positive. The Z-axis coincides with the minor axis of the spheroid, positive to the north. The Y axis is perpendicular to the XZ plane to form a right-handed system. In the survey, in order to process the observation results and transmit the coordinates of the ground control network, it is usually necessary to select a reference ellipsoid as the basic reference surface, select a reference point as the starting point of the geodetic survey (geodetic origin), and use the astronomical observation of the geodetic origin Quantities to determine the position and orientation of the reference ellipsoid within the Earth's interior.
The ginseng geodetic coordinate is widely used, and it is a general coordinate system of classical geodetic surveying. According to the map projection theory, the ginseng geodetic coordinate system can be transformed into a plane Cartesian coordinate system through Gaussian projection calculation, which provides a control basis for topographic survey and engineering survey. Because the earth ellipsoid used in different periods is different or fixed.
Both Beijing 54 and Xi'an 80 are ginseng coordinate systems.
2.2. Geocentric coordinate system
The space Cartesian coordinate system established with the center of mass of the earth as the origin, or the geodetic coordinate system established with the earth ellipsoid where the center of the sphere coincides with the center of mass of the earth as the reference plane.
A geodetic coordinate system whose origin is the center of mass of the earth. It is usually divided into a geocentric space Cartesian coordinate system (with X, Y, Z as its coordinate elements) and a geocentric geodetic coordinate system (with B, L, H as its coordinate elements). For the conversion between the space Cartesian coordinate system and the earth coordinate system, please refer to: Transformation between earth coordinates and space Cartesian coordinates . The geocentric coordinate system is an O-XYZ coordinate system established within the earth. The origin O is set at the center of mass of the terrestrial body, represented by the three axes X, Y, and Z that are perpendicular to each other. The X axis coincides with the intersection line between the Greenwich meridian and the equator, and the eastward direction is positive. The Z axis coincides with the Earth's axis of rotation, with north being positive. The Y axis is perpendicular to the XZ plane to form a right-handed system.
2.3. Background
Before the 1950s, a country or a region established its own local geodetic coordinate system according to the radian measurement method under the condition that the selected reference ellipsoid best fits the geoid of the region. Since there were only sparse gravity surveys on the ocean at that time, geodesy work could only be carried out on each continent, and there was almost no connection between the local geodetic coordinate systems of each continent. However, at the level of scientific development at that time, the local geodetic coordinate system could basically meet the requirements of geodetic surveying and cartography in various countries. However, in order to study the overall shape of the earth and its external gravitational field and geodynamic phenomena; especially after the emergence of artificial earth satellites and long-range ballistic weapons in the late 1950s, in order to describe their position and movement in space, and to represent their ground launch stations And the location of the tracking station must adopt the geocentric coordinate system. Therefore, establishing a global geocentric coordinate system (also known as a world coordinate system) has become an urgent task facing geodesy.
WGS-84 and CGCS2000 belong to the geocentric coordinate system.
3. Coordinate systems used in my country
3.1. Beijing 54 coordinate system
The Beijing 54 coordinate system (BJZ54) is a ginseng geodetic coordinate system, which is based on the Krasovsky ellipsoid and produced after local adjustment.
After the founding of New China, our country adopted the Krasovsky ellipsoid parameters of the former Soviet Union, and conducted joint measurement with the former Soviet Union's 1942 coordinate system, and established our country's geodetic coordinate system through calculation, which was named the 1954 Beijing coordinate system. Therefore, the 1954 Beijing coordinate system can be considered as an extension of the 1942 coordinate system of the former Soviet Union. Its origin is not in Beijing but in Pulkovo in the former Soviet Union . It connects the first-class lock in my country with the first-class lock in the former Soviet Union’s Far East, and then takes the coordinates of the former Soviet Union’s Pulkovo coordinate system in 1942 as the starting point for the expansion of the Huma, Jiranin, and Dongning baseline network at the connection point. , to adjust the first-class locks in the northeast and eastern regions of my country, and the coordinate system transmitted in this way is named the 1954 Beijing coordinate system.
3.2. 1980 Xi'an Coordinate System
In April 1978, the National Astronomical Geodetic Network Adjustment Conference was held in Xi'an to determine the repositioning and establish a new coordinate system in my country. For this reason, there is the 1980 National Geodetic Coordinate System. In 1980, the national geodetic coordinate system adopted the basic parameters of the earth ellipsoid as the data recommended by the 16th Congress of the International Union of Geodesy and Geophysics in 1975.
The geodetic origin of this coordinate system is located in Yongle Town, Jingyang County, Shaanxi Province in central China , about 60 kilometers northwest of Xi'an City, so it is called the 1980 Xi'an coordinate system, also referred to as the Xi'an geodetic origin.
The datum adopts the mean sea level of the Yellow Sea determined by Qingdao Dagang Tide Gauge Station from 1952 to 1979 (that is, the 1985 national elevation datum ).
3.3. Differences from Beijing 54
The Xi'an 80 coordinate system and the Beijing 54 coordinate system are actually a conversion of ellipsoid parameters, as the conversion of this conversion in the same ellipsoid is strict, but the conversion between different ellipsoids is not strict, so There is no one set of conversion parameters that can be used nationwide and will be different in each place because they are two different ellipsoidal datums.
Beijing 54 and Xi'an 80 are two different geodetic datums and different reference ellipsoids. Therefore, the coordinates of the same point are different under the two maps, whether it is the coordinates of the three-degree zone, the six-degree zone or the longitude and latitude coordinates. of.
4. CGCS2000
CGCS2000 is the 2000 national geodetic coordinate system, which belongs to the geocentric geodetic coordinate system. The system is based on the ITRF 97 reference frame, and the reference frame epoch is 2000.0.
The coordinate system is a geocentric geodetic coordinate system established through the continuous operation of the Chinese GPS reference station, the space geodetic control network, and the joint adjustment of the astronomical geodetic network and the space geodetic network. The 2000 (China) national geodetic coordinate system is based on the ITRF 97 reference frame, and the reference frame epoch is 2000.0.
The basic geodetic constants of the 2000 national geodetic coordinate system are:
Semi-major axis: a = 6378137.0 ma=6378137.0 \mathrm{~m}a=6378137.0 m
flatness:f = 1 / 298.257222101 f=1 / 298.257222101f=1/298.257222101
gravitational constant:GM = 3986004418 × GM=3986004418 \timesGM=3986004418 × 1 0 8 m 3 s − 2 10^8 \mathrm{~m}^3 \mathrm{~s}^{-2}108 m 3 s − 2
Earth rotation angular velocity:ω = 7292115.0 × 1 0 − 11 \omega=7292115.0 \times 10^{-11}oh=7292115.0×10−11 rads − 1 \operatorname{rads}^{-1} rads−1
In the 1950s, in order to meet the urgent needs of surveying and mapping, China adopted the 1954 Beijing coordinate system. After 1954, with the completion of the task of laying out the astronomical geodetic network and through the overall adjustment of the astronomical geodetic network, China established the 1980 Xi'an coordinate system in the early 1980s. With the change of the situation and the passage of time, the above two local geodetic coordinate systems based on classical surveying technology can no longer adapt to the development of science and technology, especially space technology, and cannot meet the needs of China's economic construction and national defense construction. The renewal of China's geodetic coordinate system is an objective need for economic construction, national defense construction, social development and scientific and technological development.
The geocentric geodetic coordinate system with the center of the earth's mass as the origin is the basic geodetic coordinate system used globally in the space age of the 21st century. The geocentric geodetic coordinate system based on space technology is a suitable choice for China's new generation geodetic coordinate system. The geocentric geodetic coordinate system can meet the extensive needs of geodesy, geophysics, astronomy, navigation and aerospace applications, as well as economic and social development. After many years, China's surveying and mapping, seismological departments and relevant units of the Academy of Sciences have done a lot of basic work for the establishment of China's new generation of geodetic coordinate system. At the end of the 20th century, the national GPS primary and secondary networks, the national GPS A and B-level networks, and the Chinese crustal movement The observation network and many crustal deformation networks have laid a good foundation for the realization of the geocentric geodetic coordinate system.
5. Differences between coordinate systems
5.1 Comparison between CGCS2000 and WGS-84
The definition of CGCS2000 is essentially the same as that of WGS84. The reference ellipsoid used is very close. The difference in oblateness causes the latitude and height on the ellipsoid to change up to 0.1 mm. Within the current measurement accuracy range, this difference can be ignored. It can be said that the two are compatible to the cm level, but if the coordinate accuracy of a point cannot reach the cm level, the coordinates of CGCS2000 and WGS84 are not considered compatible.
WGS-84 ellipsoid parameters
major radius a = 6378137 m \mathrm{a}=6378137 \mathrm{~m}a=6378137 m
short radiusb = 6356752.3142 m \mathrm{b}=6356752.3142 \mathrm{~m}b=6356752.3142 m
flatnessf = 1 / 298.2572236 \mathrm{f}=1/298.2572236 \mathrm{}f=1/298.2572236
G M = 3986005 × 1 0 8 m 3 s − 2 \mathrm{GM}=3986005 \times 10^8 \mathrm{~m}^3 \mathrm{~s}^{-2} GM=3986005×108 m 3 s −2
C ‾ 2.0 = − 484.16685 × 1 0 − 6 \overline{\mathrm{C}}_{2.0}=-484.16685 \times 10^{-6} C2.0=−484.16685×10− 6
ω = 7292115 × 1 0 − 11 rad/s \omega=7292115 \times 10^{-11} \mathrm{rad}/\mathrm{s}oh=7292115×10−11rad/s
5.2 Comparison between CGCS2000 and 54 and 80
CGCS2000 and 1954 or 1980 coordinate systems are fundamentally different in definition and implementation. Transformations between local coordinates and geocentric coordinates are unavoidable. Coordinate transformation is realized through joint adjustment, while one side is realized through a certain transformation model. When using model transformation, the selection of the transformation model should be based on the accuracy requirements. For high precision (better than 0.5 m 0.5m0.5 m ), the minimum curvature method or other grid models can be used, for medium precision (0.5 − 5 m 0.5-5m0.5−5 m ) requirements, the seven-parameter model can be used, for low precision (5 10 m 5~10m5 10 m ), a four-parameter or three-parameter model can be used.
6. Common ellipsoid parameters
| parameter | Semi-major axis (a/m) | Semi-minor axis (b/m) | Flatness |
|---|---|---|---|
| Krasowski ellipsoid | 6378245.0 | 6356863.01877304 | 1/298.3 |
| 1975 International Ellipsoid | 6378140.0 | 6356755.28815752 | 1/298.257 |
| WGS84 ellipsoid | 6378137.0 | 6356752.31424517 | 1/298.257223563 |
| CGCS2000 coordinate system ellipsoid | 6378137.0 | 6356752.31414036 | 1/298.25722210100 |
| GRS80 coordinate system ellipsoid | 6378137.0 | 6356752.31414036 | 1/298.25722210103 |
| PZ90 coordinate system ellipsoid | 6378136.0 | 6356751.36179569 | 1/298.25784 |
| Helmert ellipsoid parameters (1906) | 6738140.0 | 6715551.53201475 | 1/298.3 |
| Hayford ellipsoid parameters (1910) | 6378388±35 | 6356911.94612795 | 1/297.0±0.5 |
| Bessel ellipsoid parameters (1841) | 6377397±210 | 6356075.04413240 | 1/299.1±4.7 |
| Clarke ellipsoid parameters (1840) | 6378249 | 6356517.31686542 | 1/293.5 |
扁率: ∂ = a − b a \partial=\frac{a-b}{a} ∂=aa−b
First eccentricity: e = a 2 − b 2 ae=\frac{\sqrt{\mathrm{a}^2-\mathrm{b}^2}}{a}e=aa2−b2
Second eccentricity: e ′ = a 2 − b 2 be^{\prime}=\frac{\sqrt{\mathrm{a}^2-\mathrm{b}^2}}{b}e′=ba2−b2