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: The******** main examples are ISO codes: ISO 3166-1 alpha-2 code (e.g. **************** **************** for Afghanistan or **************** **************** for Brazil), and its subdivision conventions, such as AF subdivision codes **************** (e.g. **************** **************** for Ghor province) or BR subdivision codes (e.g. **************** **************** for Amazonas ********state). The ISO 19112:2019 standard (section 3.1.2) adopted the term "geographic identifier" instead geocode, to encompass long labels: spatial reference in the form of a label or code that identifies a location. For example, for ISO, the country name “People's Republic of China” is a label. 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:

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: 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, are: Note: when a physical addressing schemes (street name and house number) is expressed in a standardized and simplified way, it can be conceived as geocode. So, the term geocoding (used for addresses) sometimes is generalized for geocodes. In spatial indexing applications the geocode can also be translated between human-readable (e.g. hexadecimal) and internal (e.g. binary 64-bit unsigned integer) representations.

Systems of standard names

Geocodes like country codes, city codes, etc. comes from a table of official names, and the corresponding official codes and geometries (typically polygon of administrative areas). "Official" in the context of control and consensus, typically a table controlled by a standards organization or governmental authority. So, the most general case is a table of standard names and the corresponding standard codes (and its official geometries). Strictly speaking, the "name" related to a geocode is a toponym, and the table (e.g. toponym to standard code) is the resource for toponym resolution: is the relationship process, usually effectuated by a software agent, between a toponym and "an unambiguous spatial footprint of the same place". Any standardized system of toponym resolution, having codes or encoded abbreviations, can be used as geocode system. The "resolver" agent in this context is also a geocoder. Sometimes names are translated into numeric codes, to be compact or machine-readable. Since numbers, in this case, are name identifiers, we can consider "numeric names" — so this set of codes will be a kind of "system of standard names".

Hierarchical naming

In the geocode context, space partitioning is the process of dividing a geographical space into two or more disjoint subsets, resulting in a mosaic of subdivisions. Each subdivision can be partitioned again, recursively, resulting in an hierarchical mosaic. When subdivisions's names are expressed as codes, and code syntax can be decomposed into a parent-child relations, through a well-defined syntactic scheme, the geocode set configures a hierarchical system. A geocode fragment (associated to a subdivision name) can be an abbreviation, numeric or alphanumeric code. A popular example is the ISO 3166-2 geocode system, representing country names and the names of respective administrative subdivisions separated by hyphen. For example is Germany, a simple geocode, and its subdivisions (illustrated) are for Baden-Württemberg, for Bayern, ..., for Nordrhein-Westfalen, etc. The scope is only the first level of the hierarchy. For more levels there are other conventions, like HASC code. The HASC codes are alphabetic and its fragments have constant length (2 letters). Examples: Two geocodes of a hierarchical geocode system with same prefix represents different parts of the same location. For instance and represents geographically interior parts of , the common prefix. Changing the subdivision criteria we can obtain other hierarchical systems. For example, for hydrological criteria there is a geocode system, the US's hydrologic unit code (HUC), that is a numeric representation of basin names in a hierarchical syntax schema (first level illustred). For example, the HUC is the identifier of "Pacific Northwest Columbia basin"; HUC of "Lower Snake basin", a spatial subset of HUC and a superset of ("Imnaha River").

Systems of regular grids

Inspired in the classic alphanumeric grids, a discrete global grid (DGG) is a regular mosaic which covers the entire Earth's surface (the globe). The 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-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.

[Each cell of a regular grid is labeled by a geocode. The non-global grids were the most used before the 2000s.

This hierarchical system of local grids, used since the 1930s as British National Grid, generates hierarchical geocodes. Each cell subdivides recurrently its area into a new 10x10 grid. | upload.wikimedia.org/wikipedia/commons/f/f5/Ordnance///Survey///National///Grid.svg]

Hierarchical grids

Each cell of a grid can be transformed into a new local grid, in a recurring process. In the illustrated example, the cell is a sub-cell of , that is a sub-cell of. A system of geographic regular grid references 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: and represents geographically interior parts of , the common prefix. Hierarchical geocode can be split into keys. The Geohash is the key of the cell , that is a cell of (key ), and so on, per-digit keys. The OLC is the key of the cell , that is a cell of (key ), and so on, two-digit keys. In the case of OLC there is a second key schema, after the separator: is the key of the cell. 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 , which is a base32 code, can be expanded to base4 , 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. The uniformity of shape and area of cells in a grid can be important for other uses, like 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 OGC. When human-readable codes obtained from cell identifiers of a DGGS are also standardized, it can be classified as DGGS based geocode system.

Name-and-grid systems

There are also mixed systems, using a syntactical partition, where for example the first part (code prefix) is a name-code and the other part (code suffix) is a grid-code. Example: For mnemonic coherent semantics, in fine-grained geocode applications, the mixed solutions are most suitable.

Shortening grid-based codes by context

Any geocode system based on regular grid, in general is also a shorter way to express a latitudinal/longitudinal coordinate. But a geocode with more than 6 characters is difficult for remember. On the other hand, a geocode based on standard name (or abbreviation or the complete name) is easier to remember. This suggests that a "mixed code" can solve the problem, reducing the number of characters when a name can be used as the "context" for the grid-based geocode. For example, in a book where the author says "all geocodes here are contextualized by the chapter's city". In the chapter about Paris, where all places have a Geohash with prefix, that code can be removed —. For instance Geohash can be reduced to , or, by an explicit code for context "FR-Paris ". This is only possible when the context resolution (e.g. translation from "FR-Paris" to the prefix ) is well-known. In fact a methodology exists for hierarchical grid-based geocodes with non-variable size, where the code prefix describes a broader area, which can be associated with a name. So, it is possible to shorten by replacing the prefix to the associated context. The most usual context is an official name. Examples: The examples of the Mixed reference column are significantly easier than remembering DGG code column. The methods vary, for example OLC can be shortened by elimination of its first four digits and attaching a suitable sufficiently close locality. When the mixed reference is also short (9 characters in the second example) and there are a syntax convention to express it (suppose ), this convention is generating a new name-and-grid geocode system. This is not the case of the first example because, strictly speaking, "Cape Verde, Praia" is not a code. To be both, a name-and-grid system and also a mixed reference convention, the system must be reversible. Pure name-and-grid systems, like Mapcode, with no way to transform it into a global code, is not a mixed reference, because there is no algorithm to transform the mixed geocode into a grid-based geocode.

Cataloged examples

In use, general scope

Geocodes in use and with general scope:

In use, alternative address

Geocodes can be used in place of official street names and/or house numbers, particularly when a given location has not been assigned an address by authorities. They can also be used as an "alternative address" if it can be converted to a Geo URI. Even if the geocode is not the official designation for a location, it can be used as a "local standard" to allow homes to receive deliveries, access emergency services, register to vote, etc.

In use, postal codes

Geocodes in use, as postal codes. A geocode recognized by Universal Postal Union and adopted as "official postal code" by a country, is also a valid postal code. Not all postal codes are geographic, and for some postal code systems, there are codes that are not geocodes (e.g. in UK system). Samples, not a complete list:

In use, telephony and radio

Geocodes in use for telephony or radio broadcasting scope:

In use, others

Geocodes in use and with specific scope:

Historical or less widely used

Other examples

Other geocodes:

Other standards

Some standards and name servers include: ISO 3166, FIPS, INSEE, Geonames, IATA and ICAO. A number of commercial solutions have also been proposed: