Editing Geocode (section)

=== Systems of regular grids ===
[[file:Ordnance_Survey_National_Grid.svg|thumb|420px|Each cell of a regular grid is [[Graph labeling|labeled]] by a geocode. The non-global grids were the most used before the 2000s. <br />This ''hierarchical system of local grids'', used since the 1930s as [[Ordnance Survey National Grid|British National Grid]], generates hierarchical geocodes. Each cell subdivides recurrently its area into a new 10x10 grid.]]

{{main|Discrete global grid#Geocoding variants{{!}}DGG geocoding variants}}

Inspired in the classic [[alphanumeric grid]]s, a [[discrete global grid]] ('''DGG''') is a regular [[Tessellation|mosaic]]  which covers the entire [[Terrain|Earth's surface]] (the globe). The [[Euclidean tilings by convex regular polygons#Regular tilings|regularity of the mosaic]] is defined by the use of cells of same shape in all the grid, or "near the same shape and near same area" in a region of interest, like a country.

All cells of the grid have an identifier (DGG's cell ID), and the center of the cell can be used as reference for cell ID conversion into geographical point. When a compact human-readable expression of the cell ID is standardized, it becomes a geocode.

Geocodes of different ''geocode systems'' can represent the same position in the globe, with same shape and precision, but differ in [[String (computer science)|string]]-length, digit-alphabet, separators, etc. Non-global grids also differ by scope, and in general are geometrically optimized (avoid overlaps, gaps or loss of uniformity) for the local use.

==== Hierarchical grids ====
Each cell of a grid can be transformed into a new local grid, in a [[Recursion|recurring process]]. In the illustrated example, the cell <code>TQ 2980</code> is a sub-cell of <code>TQ 29</code>, that is a sub-cell of <code>TQ</code>. A system of geographic regular [[grid reference]]s is the base of a ''hierarchical geocode system''.

Two geocodes of a ''hierarchical geocode grid system'' can use the prefix rule: geocodes with '''same prefix''' represents different parts of the '''same broader location'''. Using again the side illustration: <code>TQ 28</code> and <code>TQ 61</code> represents geographically [[Interior (topology)|interior parts]] of <code>TQ</code>, the common prefix.

Hierarchical geocode can be split into keys. The [[Geohash]] <code>6vd23gq</code> is the key <code>q</code> of the cell <code>6vd23g</code>, that is a cell of <code>6vd23</code> (key <code>g</code>), and so on, per-digit keys. The [[Open Location Code|OLC]] <code>58PJ642P</code> is the key <code>48</code> of the cell <code>58PJ64</code>, that is a cell of <code>58Q8</code> (key <code>48</code>), and so on, two-digit keys. In the case of OLC there is a second key schema, after the <code>+</code> separator:  <code>58PJ642P+48</code> is the key <code>2</code> of the cell&nbsp;<code>58PJ642P+4</code>. It uses two key schemas. Some geocodes systems (e.g. S2 geometry) also use initial prefix with non-hierarchical key schema.

In general, as technical and non-compact optional representation, geocode systems (based on hierarchical grids)  also offer the possibility of expressing their cell identifier with a fine-grained schema, by longer path of keys. For example, the Geohash  <code>6vd2</code>, which is a [[base32]] code, can be expanded to [[Quaternary numeral system|base4]] <code>0312312002</code>, which is also a schema with per-digit keys. Geometrically, each Geohash cell is a rectangle that subdivides space recurrently into 32 new rectangles, so, base4 subdividing into 4, is the encoding-expansion limit.<ref>Note: in practical use Geohash can expand to base2, but geometrically it is based on latitude and longitude (2+2) partitions, so base2 can result in loss of symmetry. Strictly Geohash base32 also need two-digit keys for base4 compatibility.</ref>

The uniformity of shape and area of cells in a grid can be important for other uses, like [[Spatial analysis|spatial statistics]]. There are standard ways to build a grid covering the entire globe with cells of equal area, regular shape and other properties: Discrete Global Grid System (DGGS) is a series of discrete global grids satisfying all standardized requirements defined in 2017 by the [[Open Geospatial Consortium|OGC]].<ref>"Topic 21: Discrete Global Grid Systems Abstract Specification", [[Open Geospatial Consortium]] (2017). https://docs.opengeospatial.org/as/15-104r5/15-104r5.html</ref>
When human-readable codes obtained from cell identifiers of a DGGS are also standardized, it can be classified as ''DGGS based geocode system''.