## 2 Materials and Methods
### ISEA4H Subdivision Model of the Temporal and Spatial Grid of the Earth System's Layers  
#### 2.2.1 Basic Concept  
In order to clearly describe the Earth's layers in a gridded and hierarchical manner, it is necessary to first clarify the scope and concept of the Earth's layers. Intuitively, as shown in Figure 2, the layered structure of the Earth system is manifested as a series of spherical shells divided by concentric spherical surfaces with the Earth's center as the center of the sphere. These spherical shells are located between the inner and outer spherical surfaces and have a certain thickness. The innermost layer is a sphere; that is, the radius of the inner spherical surface is 0, and the center of the spatial region of the Earth system's layers is the Earth's center. For the position of any point in the spatial region of the Earth's layers, it can be represented based on the coordinates of the geocentric polar coordinate system \((\tau,\lambda,\varphi)\), where the distance from this point to the Earth's center is the radius r.  
For the entire Earth system's layered space, it can be divided into layer bodies and layer surfaces. Among them, according to the Earth's layered structure, some layer surfaces are called reference layer surfaces. Layer Body: A spherical shell defined by radius r between inner radius R and outer radius R\({}^{\prime}\) (r[R, R\({}^{\prime}\)]). Layer Surface: A spherical surface Sr at radius r from Earth's center. Layer Surface: The layer surface is a spherical surface with the Earth's center as the center of the sphere, which is used to divide different layers and can be expressed as Sr.  
Reference Layer Surface: The layer surface that divides the inner and outer layers is set according to the layer surface of the Earth's surface on which the global discrete grid is established. According to the layered structure of the Earth system, taking the reference layer surface as the boundary, it can be divided into the inner layers of the Earth, the Earth's surface, the outer layers of the Earth, etc. Among them, the inner layers include the Earth's core, mantle, crust, etc. ; the Earth's surface includes the lithosphere, hydrosphere, biosphere, etc., and the outer space includes the atmosphere, near-Earth space, etc. The result of subdividing the layered space of the Earth system is a three-dimensional space grid unit with variable layers at each subdivision level. At the same time, the layer surface is also divided into corresponding grid units. The layer-surface grid is a spherical grid formed on each layer surface during the process of subdividing the three-dimensional space of the Earth system. The subdivision units on the layer surface are called layer cells. 2.2 Design Concept of the ISEA4H-ESSG Subdivision Model of the Temporal and Spatial Grid of the Earth System's Layers  
The ISEA4H-ESSG temporal and spatial grid of the Earth system's layers needs to reflect, as much as possible, the natural spherical layered structure of the Earth system, as well as the trends in the vertical (radial) and horizontal (spherical) directions, that is, the layered structure. Therefore, the ISEA4H-ESSG should take the geocentric coordinate system as the spatial reference, use the Earth's spherical surface as the starting surface, and demarcate the reference layer surface according to the distribution characteristics of the  
Figure 2: The layered structure characteristics of the Earth system. layers of spatial activities and spatial information. The initial layer body is constructed by using the initial layer cells of the adjacent reference layer surfaces. The subdivision is carried out on the reference layer surface based on the subdivision structure of the four-aperture global discrete grid of the regular icosahedron, and the subdivided grid units are extended to the Earth's space, discretizing the entire Earth's space into three-dimensional grid units with equal areas at each subdivision level. The layer bodies of adjacent subdivision levels have a spatial subordination relationship.