# Discrete Global Grid System

DGGS - A series of discrete global grids in which the domain of interest is the earth’s surface, usually represented by some form of topologically equivalent surrogate surface, such as a sphere, a spheroid, or a geoid.

The aperture of a regular DGGS is the ratio of the areas of a cell at resolution k and at resolution k+1. One cell type typically predominates (e.g., hexagon); and the aperture of the system is defined by that dominant cell type.

**Congruent DGGS**: A DGGS in which each resolution k cell *region* consists of a union of resolution k+1 cell regions.

**Aligned DGGS**: A DGGS in which each resolution K cell *point* is also a cell point in resolution k+1.

Grids based on squares are the most familiar type of DGGS and form the basis of the latitude-longitude system (although these squares are of differing areas and become triangles at the poles). But because square grids do not exhibit uniform adjacency, they cannot be used for many applications (e.g., discrete simulations).

**Geodesic DGGS**: A DGGS that has the following properties:
1. Its base is a regular polyhedron;
2. It has a fixed orientation of its base polyhedron relative to the Earth;
3. It has a hierarchical spatial partitioning method defined symmetrically on the face of the base polyhedron.
4. It has a method for transforming that partition to a corresponding spherical or near-spherical surface; and
5. It has a method for assigning points to grid cells.