Surveying & mapping datum is a key infrastructure of national economy, social development, state security and information construction, an important foundation for determining natural geographic elements and the geometric configuration and spatial & temporal distribution of earth surface artificial facilities, and the initial numerical data for various surveying tasks and the basis reference for accurate demonstration of the geographic space distribution in the real world by making use of map. China’s surveying & mapping datum consists of geodetic datum, vertical datum, sounding datum and gravity datum.
Since its founding in 1949, with a view of meeting the requirements of economic construction, the People’s Republic of China has set up China Geodetic Coordinate System (Beijing Coordinate System, CGCS 1954), Huanghai Vertical Datum 1956 and Gravity Fundamental Network 1957, which represent the first-generation datum reference system for surveying and mapping of China.
Since 1980s, China has strengthened upgrading and reconstruction of the first-generation datum reference system for surveying and mapping and gradually built a national horizontal control network consisting of 48,000 points: namely, China Geodetic Coordinate System 1980 (Xi’an Coordinate System), which has a significantly-improved precision than CGCS 1954; it has completed the National Vertical Datum System 1985 comprised of the Class-A national leveling network with 100 loops and a total distance of 93,000 kilometers and the Class-B national leveling network with a total distance of 136,000 kilometers. Compared with the Huanghai Vertical Datum System 1956, National Vertical Datum System 1985 features higher density, higher precision and more rational structure; moreover, China has completed its National Gravity Fundamental Network 1985, which is comprised of 6 gravimetric datum points, 46 basic gravimetric points and 163 first-class gravimetric points. As the second-generation datum reference system for surveying and mapping of China, Gravity Fundamental Network 1985 features higher density and precision compared with Gravity Fundamental Network 1957.
Since 1990s, China has further accelerated the modernization of the datum reference system for surveying and mapping. In 1997, China completed its high-precision national GPS Network A and Network B, realizing the nationwide coverage with three-dimensional geocentric coordinates, 2 orders of magnitude higher than the national horizontal control network 1980. In 2003, China built the national GPS geodetic control network 2000 that is comprised of 2,500 points. In 2004, China built the national geodetic control network 2000, which consists of approximately 50,000 points and enjoys much higher positioning precision than ever. In 2003, China built the National Fundamental Gravity Network 2000, which is a new-generation national gravity datum comprised of 19 datum points and 119 fundamental points. Compared with National Fundamental Gravity Network 1985, National Fundamental Gravity Network 2000 features higher precision and more rational density and distribution of points. In 2001, China set up its first decimeter-level quasi-geoid model (CQG2000), on which the sea level elevation can be determined rapidly by making use of satellite space positioning technologies and geoid model, thereby providing a supplementary approach to traditional leveling height survey within a certain range of precision.
So far, China has basically set up a relatively complete datum reference system for surveying and mapping, including national GPS geodetic control network 2000 comprised of over 2,500 points and national geodetic control network 2000 comprised of approximately 50,000 points, as well as the national fundamental gravity network comprised of 19 datum points and 119 fundamental points, marking the start of the construction of the modern Chinese datum reference system for surveying and mapping.
Since 2006, China has further accelerated optimization and upgrading of the existing datum reference system for surveying and mapping, focused on replacing the traditional reference ellipsoid-centric and two-dimensional coordinate system with the geocentric and three-dimensional coordinate system, intensified the construction of the reference station for satellite positioning continuous operation and connected it to the international coordinate reference framework and maintained the coordinate framework through the reference station and the satellite geodetic control network; meanwhile, China has attached greater importance to acceleration of the geoid refinement, improvement of the resolution and precision of geoids as well as gradual replacement of traditional national high-level leveling; in addition, China has strived to increase the quantity of ground absolute gravimetric points, focused on speeding up the development of measurement of airborne gravity and satellite gravity and enriching gravimetric data with different scales.
On July 1, 2008, the high-precision, geocentric, three-dimensional and dynamic national geodetic coordinate system 2000, which is China’s new-generation geodetic coordinate system, was officially put into operation. Now, the state-level fundamental surveying & mapping results have been transferred to the National Geodetic Coordinate System 2000, and the existing fundamental surveying & mapping results have been or have been basically shifted to the Geodetic Coordinate System 2000 in Zhejiang, Gansu, Jiangxi, Fujian, Guangdong, Shandong, Henan, Beijing and Shanghai. The construction of satellite positioning continuous operation reference stations has been unfolded step by step. The satellite navigation positioning continuous operation reference station network with centimeter-level real-time positioning precision has been completed or is being built in 24 provinces (autonomous regions, municipalities), and over 1,200 continuous operation reference stations have been newly built. The work related to quasi-geoid refinement has been carried out in 26 provinces (autonomous regions, municipalities), of which 25 have completed the work and 23 have reached the centimeter-level precision. Based on the quasi-geoid refinement, GPS C network construction and Class-C leveling survey have been carried out in most provinces, municipalities and autonomous regions.
In June, 2012, the project of infrastructure construction of the national modern datum reference system for surveying and mapping (phase I), which is a major special surveying & mapping project of China during the 12th “Five-year Plan” period, was officially kicked off. The project is expected to, in the next 4 years, build 150 reference stations, reform 60 GNSS positioning continuous operation reference stations and make direct use of 150 reference stations, thus forming a national satellite positioning continuous operation datum network consisting of 360 reference stations; build 2,500 satellite geodetic control points, make direct use of 2000 points, thus forming a 4500-point national satellite geodetic control network, which constitutes the new-generation geodetic datum framework together with the national satellite positioning continuous operation datum network; build and rebuild 27,400 height control points, lay 110 leveling bedrock points, deploy the national Class-A leveling network with a length of 122 thousand kilometers and form the national modern vertical datum framework; deploy 50 national gravimetric datum points, improve the national gravity datum infrastructure; build 1 national surveying & mapping datum data center and form the national modern surveying & mapping datum management service system. Through the implementation of infrastructure construction of the national modern datum reference system for surveying and mapping, China plans to complete a high-precision, geocentric, dynamic, practical and uniform national datum reference system for surveying and mapping in 2015.
At the end of 2012, Beidou (COMPASS) navigation satellite system covered the most areas in the Asia-Pacific region and was officially put in commercial operation. And construction and maintenance of the modern datum reference system for surveying and mapping that is based on GNSS (GPS, GLONASS, GALILEO and COMPASS) have started in some areas in China.
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