page_title: Discrete global grid
section_title: Database modeling
hierarchical_section_title: Discrete global grid / Database modeling
caption_attribution_description: Basic (UML class representation) w:Discrete Global Grid database representation. Generated by yUML with Non-hierarchical: [GlobalGrid]++-2..*&gt;[Cell|region;centralPoint] Hierarchical: [Cell]^-[HierarchicalGrid|localGrid] [HierarchicalGrid]++-2..*&gt;[Cell] [Cell|region;centralPoint]^-[LeafCell]
context_page_description: A Discrete Global Grid is a mosaic which covers the entire Earth's surface.
  Mathematically it is a space partitioning: it consists of a set of non-empty regions that form a partition of the Earth's surface. In a usual grid-modeling strategy, to simplify position calculations, each region is represented by a point, abstracting the grid as a set of region-points. Each region or region-point in the grid is called a cell.
  When each cell of a grid is subject to a recursive partition, resulting in a "series of discrete global grids with progressively finer resolution", forming a hierarchical grid, it is named Hierarchical DGG.
  Discrete Global Grids are used as the geometric basis for the building of geospatial data structures. Each cell is related with data objects or values, or may be associated with other cells. DGGs have been proposed for use in a wide range of geospatial applications, including vector and raster location representation, data fusion, and spatial databases.
  The most usual grids are for horizontal position representation, using a standard datum, like WGS84. In this context, it is common also to use a specific DGG as foundation for geocoding standardization.
context_section_description: There are many DGGs because there are many representational, optimization and modeling alternatives. All DGG grid is a composition of its cells, and, in the Hierarchical DGG each cell uses a new grid over its local region.
  The illustration is not adequate to TIN DEM cases and similar "raw data" structures, where the database not use the cell concept (that geometrically is the triangular region), but nodes and edges: each node is an elevation and each edge is the distance between two nodes.
  In general, each cell of the DGG is identified by the coordinates of its region-point (illustrated as the centralPoint of a database representation). It is also possible, with loss of functionality, to use a "free identifier", that is, any unique number or unique symbolic label per cell, the cell ID. The ID is usually used as spatial index (such as internal Quadtree or k-d tree), but is also possible to transform ID into a human-readable label for  geocoding applications.
  Modern databases (e.g. using S2 grid) use also multiple representations for the same data, offering both, a grid (or cell region) based in the Geoid and a grid-based in the projection.
caption: In all DGG databases the grid is a composition of its cells. The region and centralPoint are illustrated as typical properties or subclasses. The cell identifier (cell ID) is also an important property, used as internal index and/or as public label of the cell (instead the point-coordinates) in geocoding applications. Sometimes, as in the MGRS grid, the coordinates make the role of ID.
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