## 2 Materials and Methods
### ISEA4H Subdivision Model of the Temporal and Spatial Grid of the Earth System's Layers
#### 2.2.1 Basic Concept
Each layer body has a unique global code and corresponds to a three-dimensional spatial region in the Earth's space, which is used to organize various information in the region of the grid unit. The layered spatial grid seamlessly covers the entire Earth's space and becomes a unified framework for various spatial information in each layer of the Earth system. Figure 3 is a conceptual diagram of the ISEA4H-ESSG layers, showing the starting surface (Earth's surface), initial layer body (spherical shell), and layer cells (hexagonal units). .2.3 Subdivision Mechanism of the ISEA4H-ESSG Temporal and Spatial Grid of the Earth System's Layers  
Unlike local planar or 3D Euclidean grids, the layered spatial grid extends spherical subdivision to near-Earth space, necessitating a distinct mechanism. The spherical-grid subdivision mechanism recursively divides a sphere (e.g., via ISEA4H's hexagonal tiling) into equal-area units [32], which we extend radially for 3D layering. The subdivision mechanism of a spatial grid is usually determined by the characteristics of the subdivided object and the subdivision requirements. For the planar and three-dimensional grid subdivision of the local Earth area, it is generally established in the coordinate system of Euclidean space indices, and the space is subdivided in each spatial dimension. The spatial subdivision mechanism of the layered spatial grid will be extended in the height range from the spherical surface to the near-Earth space based on the spherical-grid subdivision mechanism. (1) Subdivision of the Four-Aperture Hexagonal Global Discrete Grid  
The basic model to be adopted is the Icosahedral Snyder Equal-Area Projection Aperture 4 Hexagon Discrete Global Grid System (ISEA4H DGGS) [32]. For the spherical spatial grid on the Earth's surface, the IEEA4H DGGS is used to construct a multi-level  
Figure 3: A conceptual diagram of the spatial grid of the Earth’s layers. grid for spherical subdivision. The Four-Aperture Hexagonal Global Discrete Grid (ISEA4H DGGS) uses an icosahedron with four hexagonal subdivisions per parent unit. It forms ISEA4H-ESSG's surface foundation, ensuring equal-area coverage and supporting 3D extension. The object of the spherical spatial grid subdivision of the three-dimensional space of the Earth is the Earth's layered system, and the subdivision requirements are that the grid has a layered structure, seamless continuous coverage, continuous spatial distribution, and stable geometric properties. 1. Degenerate Subdivision  
Generally, people's understanding of three-dimensional space is the Euclidean space defined by three orthogonal coordinate axes, such as a cube. By regularly dividing the space along each coordinate axis, the regular subdivision of the entire three-dimensional space can be achieved, and the distribution of the subdivision units in the three spatial dimensions is uniform and consistent. The Earth's layered space is not a Euclidean space, and geometrically, the two are not homeomorphic. If we follow the idea of uniform subdivision in the Euclidean three-dimensional space, we will face the problem of a large disparity in the granularity of the grid units; that is, the closer to the Earth's center, the smaller the volume of the grid units and the denser the spatial distribution; the farther from the Earth's center, the larger the volume of the grid units and the sparser the spatial distribution. In order to avoid this centripetal convergence effect of the grid units in spatial distribution, the design of the layered spatial grid takes into account the degenerate processing of the grid units. That is, in the hierarchical subdivision of the Earth's layered space, among the sub-layer bodies subdivided from the same layer body, multiple sub-layer bodies close to the Earth's center will be \"merged\" and thus \"degenerate\" into one sub-layer body. The result of the degenerate subdivision of the layered spatial grid is that the granularity of the layered spatial grid units and their distribution in the Earth's space are more uniform.