Geocode System

A geocode is a code that represents a geographic entity (location or object). It is a unique identifier of the entity, to distinguish it from others in a finite set of geographic entities. In general the ''geocode'' is a human-readable and short identifier.

Typical geocodes and entities represented by it:
* ''Country code'' and subdivision code. Polygon of the administrative boundaries of a country or a subdivision.
The main examples are ISO codes: ISO 3166-1 alpha-2 code (e.g. AF for Afghanistan or BR for Brazil), and its subdivision conventions, such as subdivision codes (e.g. AF-GHO for Ghor province) or subdivision codes (e.g. BR-AM for Amazonas state).
* ''DGG cell ID''. Identifier of a cell of a discrete global grid: a Geohash code (e.g. ~0.023 km² cell 6vjyngd at the Brazilian's center) or an OLC code (e.g. ~0.004 km² cell 58PJ642P+4 at the same point).
* ''Postal code''. Polygon of a postal area: a CEP code (e.g. 70040 represents a Brazilian's central area for postal distribution).

Geocodes are mainly used (in general as an atomic data type) for labelling, data integrity, geotagging and spatial indexing.

In theoretical computer science a ''geocode system'' is a locality-preserving hashing function.

Classification

There are some common aspects of many geocodes (or geocode systems) that can be used as classification criteria:
* ''Ownership'': proprietary or free, differing by its licences.
* ''Formation'': the geocode can be originated from a name (ex. abbreviation of official name the country) or from mathematical function (encoding algorithm to compress latitude-longitude). See ''geocode system'' types below (of names and of grids).
* ''Covering'': global or partial. The entities (represented by the geocodes) are in all globe (e. g. geographical points) or is delimited the theme (e.g. only terrestrial areas) or by the ownership's jurisdiction (e.g. only into a country).
* ''Type of the represented entity'': type of geometry. Point (the geocode can be translated to a Geo URI), grid cell (the geocode system is related with a DGG) or polygon (typically administrative boundaries delimitations).

** special hierarchical grids, with global covering and equal-area cells, can be classified as ''DGGS cell''The OGS's standard
Discrete Global Grid Systems
definition.
** some non-standard geographic entities, can be classified also by its coordinate system and elipsoid of reference (e.g. UTM). The ''de facto'' standard is the WGS84.
* ''Scope of use'': general use vs specialized (e.g. airport geocodes).
* ''Hierarchy'': geocode's syntax hierarchy corresponding to the spatial hierarchy of its represented entities. A geocode system can hierarchical ( name or grid) or non-hierarchical.

Geocode system

The set of all geocodes used as unique identifiers of the cells of a full-coverage of the geographic surface (or any well-defined area like a country or the oceans), is a geocode system (also named ''geocode scheme''). The syntax and semantic of the geocodes are also components of the system definition:
* geocode syntax: the characters that can be used, blocks of characters and its size and order. Example: country codes use two letters of the alphabet (chacacter set A-Z). The most commom way to describe formally is by regular expression (e.g. / -Z).
* geocode semantic: the meaning of the geocode, usually expressed by associating the code with a geographical entity type. Can be described formally is by an ontology, an UML class diagram or any Entity-relationship model.
In general the semantic can be deduced by its formation or encoding/decoding process. Example: each Geohash code can be expressed by a rectangular area in the map, and the rectangle coordinates is obtained by its decoding process.

Many syntax and semantic characteristics are also summarized by classification.

Encode and decode

Any geocode can be translated from a formal (and expanded) expression of the geographical entity, or vice versa, the geocode translated to entity. The first is named encode process, the second decode. The actors and process involved, as defined by OGC,Definitions of the OGC's
Glossary of Terms
. are:

;geocoder: A software agent that transforms the description of a geographic entity (e.g. location name or latitude/longitude coordinates), into a normalized data and encodes it as a geocode.

;geocoder service: A geocoder implemented as web service (or similar service interface), that accepts a set of geographic entity descriptors as input. The request is "sent" to the Geocoder Service, which processes the request and returns the resulting geocodes. More general services can also return geographic features (e.g. GeoJSON