Like air and water, geomagnetic field is one of the indispensable environmental conditions for human survival. Like a natural barrier, the geomagnetic field surrounding the Earth protects the life on Earth from the threat of high temperature, high speed solar winds and the bombardment of cosmic high-energy particles. The geomagnetic field originates from the magnetic fluid process of the outer core of the Earth and the current system of the solar system, so the geomagnetic field carries rich information about the inner and outer spaces of the Earth.^1–2

Development of geomagnetism depends on the long-term accumulation of data. Geomagnetism is closely related to national economy and national defense construction. The research outcomes of geomagnetism directly or indirectly serve many fields, including navigation, communication, space environment monitoring, the forecast of natural disasters and climate changes, the exploration of energy and mineral resources, and so forth. A complete high-precision geomagnetic data set not only serves geomagnetism itself, but also provides the basis for research on, for example, seismics, geological structure, deep earth exploration and mineral exploration.^3–5

In the study of geomagnetics and geospatial environments, ground station observation plays an important role. Study of the spatial distribution and long-term variation of the main magnetic field of the Earth mainly relies on data from geomagnetic station and geomagnetic complex points. In the study of geomagnetic origin and long-term variations of the main magnetic field, satellite magnetic data still cannot replace ground data. Indeed, studies of the origin of magnetic fields and the internal processes of the Earth have been drawing basic data from over-100-year station observation data, as well as hundreds-of-year sea magnetic data.^6–8 With a large number of high-temporal resolution and high-precision geomagnetic data, scientists have made significant progress in sub-storm current systems, ionospheric-magnetospheric coupling and spatial weather forecasting. The long-term historical observation data provide a solid foundation for these studies.^8–10 After decades of efforts, China has basically completed a nationwide digital geomagnetic observation network composed by 160 observation stations, which achieved the quasi-real-time transmission and automatic collection of observation data, and a large number of observation data have been accumulated since then.¹¹

Beijing Ming Tombs station (BMT) was established in October 1985 and completed in August 1990. The station belongs to the Institute of Geology and Geophysics, Chinese Academy of Sciences. It is the first observation station entering the International Real-time Magnetic Observatory Network (INTERMAGNET). The station is located at about 600 meters northeast of Deshengkou Village, Changping District, Beijing. The geological base of this station is quartzite and gray matter rock. The magnetic background has a small local anomaly. After the land leveling and reconstruction, the distribution of the geomagnetic field has improved, in which the gradient of the total intensity of the magnetic field in the recording room and its surrounding area is less than 1 nT/m.

The geomagnetic observation room was built above ground. To ensure a stable instrument base, the foundation of external walls and 13 indoor white marble instrument frusta was built using limestone from 4 meters deep. There are three permanent signs in the southwest of the observation room, of which the commonly used standard sign No. 2 is about 700 meters from the observation room and has an azimuth angle of 207º31'50". The geomagnetic recording room is a semi-underground building. Its indoor temperature difference is less than 0.5℃, annual temperature difference about 12℃, and relative humidity less than 70%.¹²

Raw data observed by the Beijing Ming Tombs station are transmitted to the Data Center of the Space Environment Exploration Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences. Based on the raw data, the curves are drawn on server of the Data Center to illustrate the changes of geomagnetic components. The real-time curves are published online by the website of the laboratory (http://space.iggcas.ac.cn). At the beginning of the second year, the automatic observation data of the previous year are processed manually by the laboratory personnel. The processing includes to compare and correct a variety of instrument data, and to calculate the baseline value and scale value using artificial observation data as well. Finally, the IAGA2002 format data are generated and published online through the website of WDC for Geophysics, Beijing (http://wdc.geophys.ac.cn). The flow chart of geomagnetic data processing is shown in Figure 1.

Data collection is mainly operated by computer programs automatically, which have been running stably since 1991. To ensure the reliability of the data, the following measures are adopted for quality control.

(1) In order to ensure the time accuracy of the system, the clock of the acquisition system is manually adjusted periodically.

(2) In order to ensure the reliability of data transmission, all uploaded data files are validated against the raw data, and erroneous data are retransmitted.

(3) In order to ensure the quality of data, we use multiple sets of instruments to record, and through the comparison, to eliminate invalid data.

This dataset can be used to study geomagnetic field, space environment and the coupling relationship among various Earth systems. The data set contains minute values whereas the observed raw data contains second values. Raw data can be obtained by contacting the authors.