# Chinese Geomagnetic Reference Field 2020 by the Revised Surface Spline Method
## 1 Introduction  
As a standard geomagnetic product, the geomagnetic reference field provides a higher-precision geomagnetic description field of the region. A geomagnetic reference field can easily provide the geomagnetic normal reference value of the target location so as to help identify whether the measurement data are abnormal and the magnitude of the anomaly. Therefore, in the field of energy and mineral resources, geomagnetic reference fields are needed to provide the normal distribution of the geomagnetic field so as to help accurately estimate the geomagnetic anomaly morphology and predict the mineral locations and reserves. By analyzing the precise geomagnetic field distribution provided by the geomagnetic reference field itself, the structure distribution of underground materials can be inferred, thus providing help in tectonic-geodynamic studies. The high-precision geomagnetic reference field also provides accurate and undisturbed navigation information for archaeological and environment investigations, military aviation, and other fields [1; 2; 3; 4; 5]. One important research problem with geomagnetism is how to describe the spatial and temporal distribution of the geomagnetic field as accurately as possible. To describe the global geomagnetic field and its secular variation, the International Association of Geomagnetism and Aeronomy (IAGA) has issued the International Geomagnetic Reference Field (IGRF) once every 5 years since 1968, with few exceptions [6]. The latest model, the thirteenth generation of IGRF, IGRF13, was issued in December 2019 [7]. While the IGRF is intended to describe the global geomagnetic field, its usefulness in characterizing the geomagnetic field over a regional area is limited due to its higher errors. Various countries,therefore, establish national geomagnetic charts or models according to the needs of the scientific community [8; 9; 10]. National geomagnetic charts, which describe the spatial-temporal variation in the regional geomagnetic field, tend to be country wide and are established or published every 5-10 years usually [11; 12; 13; 14; 15; 16; 17]. These charts or models describe the spatial distribution of the geomagnetic field of a region and are based on some absolute geomagnetic data. Efforts to develop such charts are faced with the technical challenge of describing the spatial distribution of the geomagnetic field with accuracy and proper resolution based on a large amount of spatially discrete data. Building a model and then calculating the regional geomagnetic field is an efficient method to resolve this technical problem. The first generation of the Chinese geomagnetic charts, i.e., China geomagnetic chart for 1950.0, was established in the 1950s. Seven generations of China geomagnetic charts were published during 1960-2000 (every 10 years). These charts were generated mainly using the Taylor polynomial method [18; 19; 20]. After that, China geomagnetic charts have been made using the spherical cap harmonic (SCH) method [21; 22; 23; 24; 25; 26; 27; 28; 29]. The SCH method is derived from the spherical harmonic method used in the International Geomagnetic Reference Field (IGRF) and meets the potential theory of the geomagnetic field while providing the three-dimensional structure of the field. Despite the advantages of the SCH method, it has its own disadvantages. In addition to its being a little complex to calculate the non-integer-order Schmitt-Legendre functions, we find the non-integral-order Schmitt-Legendre function divergent oscillation when we increase the truncation order to improve the description precision. [PERSON] tried to use three sets of low-order spherical cap harmonic coefficients to describe the geomagnetic field in China to solve this problem [30]. However, it is necessary to first determine which set of coefficients should be chosen, according to the locations, and there are some complex steps to determine the geomagnetic field where the three models overlap.