## 2 Materials and Methods
### Requirements for the Earth-System Spatial Grid
#### 2.1.2 Principles and Requirements for the Earth-System Spatial Grid  
In 1994, [PERSON] was the first to propose the basic criteria for the division of the Discrete Global Grid (DGG), which include shape consistency. That is, the shapes of the grid cells should be kept as consistent as possible to reduce the errors generated during analysis and processing due to shape differences. It also includes recursive subdivisibility, which enables the grid to be recursively subdivided according to specific rules, meeting the requirements of data processing with different precisions and facilitating the effective description of the geographical space at different scales. Geographical correspondence is another criterion, ensuring that there is a clear correspondence between the grid and geographical coordinates, making it convenient for the conversion of geographical data between the traditional coordinate system and the grid system [30]. In 1999, [PERSON] improved on [PERSON]'s research findings and established the Kimerling standard [31]. This standard covers several core elements: multi-level nature; area approximation; shape approximation; universality; recognizability; and simplicity of conversion with latitude and longitude coordinates. The Kimerling standard is also regarded as the core of the evaluation criteria for DGG division. However, this standard has limitations when facing the complex multi-layered structure of the Earth system, massive dynamic data, and special research requirements. The Kimerling standard excels for 2D grids but lacks criteria for 3D vertical stratification, dynamic multi-layer data fusion, and real-time adaptability required by Earth-system science [31]. In order to describe the Earth system more comprehensively and accurately, meet the unique needs of cross-layer, multi-scale, dynamic change research, and multi-source data-fusion analysis, and promote the in-depth development of Earth-system science research, this paper, based on the above ideas, proposes the six criteria of ESSG (Earth-System Space Grid) for the Earth-system space. 1. Stratified Spherical Coverage Criterion This criterion mandates seamless, gap-free grid coverage across all volumetric layers, extending from Earth's surface to the lithosphere and upward to the ionosphere. Closely related to [PERSON]'s \"Global Coverage\" principle, this criterion expands its scope from 2D surface coverage to 3D volumetric space. For example, in seismic studies, grids must penetrate the lithosphere to precisely map seismic wave propagation and energy release, aiding earthquake prediction. In atmospheric research, grids must vertically span from the troposphere to the stratosphere to track ozone, water vapor, and greenhouse gas dynamics, supporting climate studies and pollution control. 2. Geographic Consistency Criterion Geographic consistency refers to the alignment of Earth-system phenomena with latitude-longitude distributions, reflecting natural geographic patterns (e.g., topography, climate zones, ocean currents). Earth's rotation and revolution further drive latitudinal and seasonal variations in solar radiation, influencing climate, hydrology, biology, and soil distribution. Human activities and existing geographic datasets (largely based on latitude-longitude systems) also reinforce this spatial cognition. This criterion requires ESSG to align with Earth's natural features, human cognitive habits, and existing data frameworks. Grid boundaries should align with meridians, parallels, or key geographic boundaries to intuitively reflect phenomena locations. Compatibility with latitude-longitude systems minimizes data conversion costs, enhances processing efficiency, and improves usability. ESSG must enable bidirectional conversion between grid cells and geographic coordinates. 3.