Ageocode is acode that represents a geographic entity (location orobject). It is aunique identifier of the entity, to distinguish it from others in afinite set of geographic entities. In general thegeocode is ahuman-readable and short identifier.

Typical geocodes (in bold) 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 forAfghanistan orBR forBrazil), and its subdivision conventions, such asAF subdivision codes (e.g.AF-GHO forGhor province) orBR subdivision codes (e.g.BR-AM forAmazonas state).

• DGG cell ID. Identifier of a cell of adiscrete global grid: aGeohash code (e.g. ~0.023 km² cell6vd23gq at Brazil'scentroid) or aPlus Code (e.g. ~0.0002 km² cell58Q8XXXX+XX within the same area).

• Postal code. Polygon of apostal area: aCEP code (e.g.70040 represents a Brazilian's central area for postal distribution).

The ISO 19112:2019 standard^[1] (section 3.1.2) adopted the term "geographic identifier" insteadgeocode, 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. Some authors, such as the United States Census Bureau,^[2] use the abbreviation "GEOID" as a synonym forgeocode.

Geocodes are mainly used (in general as anatomic data type) forlabelling,data integrity,geotagging andspatial indexing.

Intheoretical computer science ageocode system is alocality-preserving hashing function.

There are some common aspects of many geocodes (orgeocode systems) that can be used as classification criteria:

• Ownership: proprietary orfree, differing by itslicences.
• Formation: the geocode can be originated from a name (ex. abbreviation of official name the country) or from mathematical function (encoding algorithm to compresslatitude-longitude). Seegeocode system types below (ofnames and ofgrids).
• Hierarchy: geocode's syntax hierarchy corresponding to the spatial hierarchy of its represented entities. A geocode system can hierarchical (name orgrid) or non-hierarchical.
• 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 aGeo URI), grid cell (the geocode system is related with aDGG) or polygon (typically administrative boundaries delimitations).
  • special hierarchical grids, with global covering and equal-area cells, can be classified asDGGS cell^[3]
  • some non-standard geographic entities, can be classified also by itscoordinate system and elipsoid of reference (e.g.UTM). Thede facto standard is theWGS84.^[4]
• Scope of use: general use vs specialized (e.g. airport geocodes).

The set of all geocodes used as unique identifiers of the cells of afull-coverage of thegeographic surface (or any well-defined area like a country or the oceans), is ageocode system (also namedgeocode scheme). Thesyntax andsemantic of the geocodes are also components of the system definition:

• geocodesyntax: 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 common way to describe formally is byregular expression (e.g./[A-Z]{2,2}/).
• geocodesemantic: the meaning of the geocode, usually expressed by associating the code with a geographical entity type. Can be described formally is by anontology, anUML class diagram or anyEntity-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.

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 namedencode process, the seconddecode. The actors and process involved, as defined byOGC,^[5] are:

geocoder

Asoftware 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 asweb 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 object) represented by the geocodes.

geocoding

Geocoding refers to the assignment of geocodes or coordinates to geographically reference data provided in a textual format. Examples are the two letter country codes and coordinates computed from addresses.
Note: when aphysical addressing schemes (street name and house number) is expressed in a standardized and simplified way, it can be conceived as geocode. So, the termgeocoding (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.

Geocodes likecountry 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 astandards organization or governmental authority. So, the most general case is a table ofstandard names and the correspondingstandard codes (and its official geometries).

Strictly speaking, the "name" related to a geocode is atoponym, and the table (e.g. toponym to standard code) is the resource fortoponym resolution: is therelationship process, usually effectuated by a software agent, between a toponym and "an unambiguous spatial footprint of the same place".^[6] Any standardized system of toponym resolution, having codes or encoded abbreviations, can be used asgeocode system. The "resolver" agent in this context is also ageocoder.

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".

In the geocode context,space partitioning is the process of dividing ageographical space into two or moredisjointsubsets, resulting in amosaic 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 theISO 3166-2 geocode system, representing country names and the names of respectiveadministrative subdivisions separated by hyphen. For exampleDE isGermany, a simple geocode, and its subdivisions (illustrated) areDE-BW forBaden-Württemberg,DE-BY forBayern, ...,DE-NW forNordrhein-Westfalen, etc. The scope is only the first level of the hierarchy. For more levels there are other conventions, like HASC – Hierarchical Administrative Subdivision Codes.^[7][8] The HASC codes are alphabetic and its fragments have constant length (2 letters). Examples:

DE.NW -North Rhine-Westphalia. A two-level hierarchical geocode.

DE.NW.CE - KreisCoesfeld. A 3-level hierarchical geocode.

Two geocodes of ahierarchical geocode system with same prefix represents different parts of the same location. For instanceDE.NW.CE andDE.NW.BN represents geographically interior parts ofDE.NW, the common prefix.

Changing thesubdivision criteria we can obtain other hierarchical systems. For example, forhydrological criteria there is a geocode system, the US'shydrologic unit code (HUC), that is a numeric representation ofbasin names in a hierarchical syntax schema (first level illustred). For example, the HUC17 is the identifier of "Pacific Northwest Columbia basin"; HUC1706 of "Lower Snake basin", a spatialsubset of HUC17 and a superset of17060102 ("Imnaha River").

Inspired in the classicalphanumeric grids, adiscrete global grid (DGG) is a regularmosaic which covers the entireEarth's surface (the globe). Theregularity 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 differentgeocode systems can represent the same position in the globe, with same shape and precision, but differ instring-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 grid can be transformed into a new local grid, in arecurring process. In the illustrated example, the cellTQ 2980 is a sub-cell ofTQ 29, that is a sub-cell ofTQ. A system of geographic regulargrid references is the base of ahierarchical geocode system.

Two geocodes of ahierarchical geocode grid system can use the prefix rule: geocodes withsame prefix represents different parts of thesame broader location. Using again the side illustration:TQ 28 andTQ 61 represents geographicallyinterior parts ofTQ, the common prefix.

Hierarchical geocode can be split into keys. TheGeohash6vd23gq is the keyq of the cell6vd23g, that is a cell of6vd23 (keyg), and so on, per-digit keys. TheOLC58PJ642P is the key48 of the cell58PJ64, that is a cell of58Q8 (key48), and so on, two-digit keys. In the case of OLC there is a second key schema, after the+ separator:58PJ642P+48 is the key2 of the cell 58PJ642P+4. 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 Geohash6vd2, which is abase32 code, can be expanded tobase40312312002, 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.^[9]

The uniformity of shape and area of cells in a grid can be important for other uses, likespatial 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 theOGC.^[10]When human-readable codes obtained from cell identifiers of a DGGS are also standardized, it can be classified asDGGS based geocode system.

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:

Mapcode entrance to the elevator of theEiffel Tower in Paris isFR-4J.Q2, whereFR is the name-code^[11] and4J.Q2 is the grid-code. Semantically France is the context, to obtain its local grid.

Formnemonic coherent semantics, in fine-grained geocode applications, the mixed solutions are most suitable.

Anygeocode systembased 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, ageocode 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 prefixu09, that code can be removed —. For instance Geohashu09tut can be reduced totut, or, by an explicit code for context "FR-Paristut". This is only possible when the context resolution (e.g. translation from "FR-Paris" to the prefixu09) is well-known.

In fact a methodology exists forhierarchical 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 theMixed reference column are significantly easier than rememberingDGG 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.^[12]

When the mixed reference is also short (9 characters in the second example) and there are a syntax convention to express it (suppose CP‑PR~bgxed), this convention is generating a newname-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, likeMapcode, 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.

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Geocodes in use and with general scope:

Geocodes can be used in place of officialstreet names and/orhouse 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 aGeo 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.

Geocodes in use, aspostal codes. A geocode recognized byUniversal Postal Union and adopted as "official postal code" by acountry, 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:

Geocodes in use for telephony or radio broadcasting scope:

• ITU-R country codes
• ITU-T country calling codes
• ITU-T mobile calling codes
• Maidenhead Locator System (used by amateur radio operators)
• Marsden Squares

Geocodes in use and with specific scope:

Other geocodes:

• S2: a geocoding scheme usingspherical geometry and the space-fillingHilbert curve, developed atGoogle^[20][21]
• H3: Hexagonal Hierarchical Spatial Index a geocoding scheme initially developed at Uber^[22] source code available^[23] and documented at h3geo^[24]
• Munich Orientation Convention: converts lat/lon to metrical monopolar codes for targets, crossings, stations, stop points, bridges, tunnels, towns, islands, volcanoes, highway exits etc.^[25]
• SALB (Second Administrative Level Boundaries), by UN^[26]
• OpenPostcode, opensource global algorithm (local adaptations as Irish & Hong Kong postcodes).^[27]
• WOEID
• OpenStreetMap shortlink, used as a short permanent link to map locations^[28]
• Quarter Degree Grid Cells
• NAC (patended), area codes (area can be indefinitely small)
• GEOID, the name ofUnited States Census Bureau geographic identifiers.^[29]
• In the United States, theAmerican National Standards Institute (ANSI) Codes are often used. ANSI INCITS 446-2008 is entitled "Identifying Attributes for Named Physical and Cultural Geographic Features (Except Roads and Highways) of the United States, Its Territories, Outlying Areas, and Freely Associated Areas, and the Waters of the Same to the Limit of the Twelve-Mile Statutory Zone".
• National Topographic System in Canada
• ONS coding system (hierarchical code used in the UK for tabulating census)

Some standards and name servers include: ISO 3166, FIPS, INSEE, Geonames, IATA andICAO.

A number of commercial solutions have also been proposed:

• WOEID (Where on Earth IDentifier) is a unique 32-bit reference identifier that identifies any feature on Earth.
• NAC Locator provides a universal geocoding address for all locations on the planet.

• Census tract
• Geolocation
• Geotagging
• Geographic information retrieval
• Global Navigation Grid Code (China geocode?)
• ISO 6709, standard representation of geographic point location by coordinates
• Place code
• Unique Property Reference Number
• Unique Street Reference Number

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• Geocodes

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