Geocode

Not to be confused withAddress geocoding.

Ageocodeis acodethat represents a geographic entity (locationorobject). It is aunique identifierof the entity, to distinguish it from others in afinite setof geographic entities. In general thegeocodeis ahuman-readableand short identifier.

Typical geocodes and entities represented by it:

• Country codeand subdivision code. Polygon of the administrative boundaries of a country or a subdivision.
  The main examples are ISO codes:ISO 3166-1 Alpha -2code (e.g.AFforAfghanistanorBRforBrazil), and its subdivision conventions, such asAFsubdivision codes(e.g.AF-GHOforGhor province) orBRsubdivision codes(e.g.BR-AMforAmazonas state).
• DGG cell ID.Identifier of a cell of adiscrete global grid:aGeohashcode (e.g. ~0.023 km²cell6vjyngdat theBrazilian's center) or anOLCcode (e.g. ~0.004 km²cell58PJ642P+4at the same point).
• Postal code.Polygon of apostal area:aCEPcode (e.g.70040represents a Brazilian's central area for postal distribution).

TheISO 19112:2019standard (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 anatomic data type) forlabelling,data integrity,geotaggingandspatial inde xing.

Intheoretical computer scienceageocode systemis alocality-preserving hashing function.

Classification

Geocode cells ofGeohash,with 8 (blue) and 9 (yellow) digits, a typicalhierarchical grid,comparing withlatitude-longitude(12 or more digits). Amuseumis a typical location to be pointed by a geocode, its gate need ~20 meters of precision.

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 (encodingalgorithm to compresslatitude-longitude). Seegeocode systemtypes below (ofnamesand ofgrids).
• Hierarchy:geocode's syntax hierarchy corresponding to the spatial hierarchy of its represented entities. A geocode system can hierarchical (nameorgrid) 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
  • some non-standard geographic entities, can be classified also by itscoordinate system and elipsoid of reference(e.g.UTM). Thede factostandardis theWGS84.
• Scope of use:general use vs specialized (e.g. airport geocodes).

Geocode system

The set of all geocodes used as unique identifiers of the cells of afull-coverageof thegeographic surface(or any well-defined area like a country or the oceans), is ageocode system(also namedgeocode scheme). Thesyntaxandsemanticof 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 Alpha bet (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 diagramor 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.

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 namedencodeprocess, the seconddecode.The actors and process involved, as defined byOGC,are:

geocoder

Asoftware agentthat 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.GeoJSONobject) 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 inde xing 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

Main article:Toponym resolution

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 organizationor governmental authority. So, the most general case is a table ofstandard namesand the correspondingstandard codes(and its official geometries).

Germany (DE) with eachfirst-level administrative subdivisionlabelled with the second part of its ISO 3166-2 code.

The 21 top-level 2-digit "region" of hydrologic unit boundaries, using the HUC geocode conventions.

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".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".

Hierarchical naming

In the geocode context,space partitioningis the process of dividing ageographical spaceinto two or moredisjointsubsets,resulting in amosaicof 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 Alpha numeric code.

A popular example is theISO 3166-2geocode system, representing country names and the names of respectiveadministrative subdivisionsseparated by hyphen. For exampleDEisGermany,a simple geocode, and its subdivisions (illustrated) areDE-BWforBaden-Württemberg,DE-BYforBayern,...,DE-NWforNordrhein-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 Alpha betic 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 systemwith same prefix represents different parts of the same location. For instanceDE.NW.CEandDE.NW.BNrepresents geographically interior parts ofDE.NW,the common prefix.

Changing thesubdivision criteriawe can obtain other hierarchical systems. For example, forhydrological criteriathere is a geocode system, the US'shydrologic unit code(HUC), that is a numeric representation ofbasin namesin a hierarchical syntax schema (first level illustred). For example, the HUC17is the identifier of "Pacific Northwest Columbia basin";HUC1706of "Lower Snake basin",a spatialsubsetof HUC17and a superset of17060102( "Imnaha River" ).

Systems of regular grids

Each cell of a regular grid islabeledby a geocode. The non-global grids were the most used before the 2000s.
Thishierarchical system of local grids,used since the 1930s asBritish National Grid,generates hierarchical geocodes. Each cell subdivides recurrently its area into a new 10x10 grid.

Main article:DGG geocoding variants

Inspired in the classicAlpha numeric grids,adiscrete global grid(DGG) is a regularmosaicwhich covers the entireEarth's surface(the globe). Theregularity of the mosaicis 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 systemscan represent the same position in the globe, with same shape and precision, but differ instring-length, digit- Alpha bet, 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 arecurring process.In the illustrated example, the cellTQ 2980is a sub-cell ofTQ 29,that is a sub-cell ofTQ.A system of geographic regulargrid referencesis the base of ahierarchical geocode system.

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

Hierarchical geocode can be split into keys. TheGeohash6vd23gqis the keyqof the cell6vd23g,that is a cell of6vd23(keyg), and so on, per-digit keys. TheOLC58PJ642Pis the key48of 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+48is the key2of the cell58PJ642P+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 abase32code, 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.

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. When human-readable codes obtained from cell identifiers of a DGGS are also standardized, it can be classified asDGGS 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:

Mapcodeentrance to the elevator of theEiffel Towerin Paris isFR-4J.Q2,whereFRis the name-codeand4J.Q2is the grid-code. Semantically France is the context, to obtain its local grid.

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

Shortening grid-based codes by context

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 Geohashu09tutcan 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 geocodeswith 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:

Standards mixed	Grid-based	Mixed reference
GridOLCand country's official names	796RWF8Q+WF	Cape Verde, Praia,WF8Q+WF
GridGeohashandISO 3166-2 hierarchical abbreviations	e6xkbgxed	CV-PR,bgxed

The examples of theMixed referencecolumn are significantly easier than rememberingDGG codecolumn. 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 (supposeCP‑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.

Cataloged examples

In use, general scope

Geocodes in use and with general scope:

Geocode	Inception	Coverage	Formation	Ownership	Rep. entity	Context and description
ISO 3166 (Alpha -2andAlpha -3)	1974	globe/only nations	Name abbreviation	free	polygon	Administrative divisions. Country codes and codes of their subdivisions. Two letters ( Alpha -2) or three letters ( Alpha -3).
ISO 3166-1 numeric	1970	globe/only nations	Serial number	free	polygon	Administrative divisions. Country codes expressed by serial numbers.
UN M.49	~1970	globe/only nations	Serial number	free	polygon	Administrative divisions. region codes, area code, continents, countries (re-using ISO 3166-1 numeric codes).
Geohash	2008	globe	encode(latLon,precision)	free	grid cell	Hashnotation for locations. See alsoGeohashand its variants, likeOpenStreetMap'sshort-link
Open Location Code(OLC)	2014	globe	encode(latLon,precision)	free	grid cell	See alsoPlusCodes.
What3words	2013	globe	encode(latLon)	patented	grid cell	patent-restrictions system, converts 3x3 meter squares into 3 words.It is in use atMongol Post.
Mapcode	2001	globe	encode(latLon)	patented	point	A mapcode is a code consisting of two groups of letters and digits, separated by a dot.
Geopeg	2020	globe/only nations	encode(latLon)	open standard	grid cell	Geopeg is word-based GPS address, using simple words like London.RedFish. It is a combination of a city and two simple words. It is an open standard geocoding of Earth, currently in development.Geopeg

In use, alternative address

Geocodes can be used in place of officialstreet namesand/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.

Geocode	Inception	Coverage	Formation	Ownership	Rep. entity	Context and description
Local OLC(Cape Verde)	2016	globe	encode(latLon,precision)	free	grid cell	OLC is used to provide postal services.
Eircode(Ireland)	2014	Ireland	encode(latLon,precision)	copyrighted	grid cell	It is used officially asalternative addressand aspostal code.Limited database and algorithm access. It is a kind offine-grained postal code.

In use, postal codes

Geocodes in use, aspostal codes.A geocode recognized byUniversal Postal Unionand 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:

Geocode	Inception	Coverage	Formation	Ownership	Rep. entity	Context and description
CEP(Brazil)	1970?	cities or streets	Hierarchical serial number	proprietary	(variable)	... The CEP5 is geographic and CEP8 can be a city (polygon), a street (also street side or a fragment of street side) or a point (specific address).
Postal Index Number(India)	?	postal regions	Hierarchical serial number?	proprietary?	(undefined?)	...
ZIP Code(United States)	?	postal regions	Hierarchical serial number?	proprietary?	(undefined?)	...

In use, telephony and radio

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

In use, others

Geocodes in use and with specific scope:

Geocode	Inception	Scope	Coverage	Formation	Ownership	Rep. entity	Context and description
ONScode	2001	UKonly	UK/themes	Serial number	free	polygon	Administrative divisions. Geographical areas of the UK, for use in tabulating census.
NUTSarea code	2003	EUonly	Europe	Hierarchical	free	polygon	Administrative divisions. Partially administrative, worldwide (countries) and Europe (country to community)
MARC country codes	1971	USA only?	globe/only nations	Name abbreviation	free	polygon	Administrative divisions. Country codes.
SGC codes	?	Canadaonly	?	Serial number	free	polygon	Administrative divisions, numeric codes.... Statistical, like ONS.
UN/LOCODE	?	trade and transport	globe	Serial number	free	polygon	Administrative divisions.UNcodes for trade and transport locations.
IATA airport codes	1930s	airport	globe	?	free	polygon	Administrative divisions. area /point codes, airports and 3-letter city codes
ICAO airport codes	1950s	airport	globe	?	free	polygon	Administrative divisions.area /point codes, airports
IANA country codes	1994	Internet	globe	?	free	polygon	Administrative divisions. Similar toISO 3166-1 Alpha -2,seeCountry code top-level domain,ListandInternationalized country codes.
IOC country codes	~1960	Sport	globe	abbreviation	free	polygon	Administrative divisions. Codes ofIOCmembers; uses three-letter abbreviation country codes, like ISO 3166-1 Alpha -3.
Longhurst code	?	Environment	globe	?	free	polygon	Administrative divisions. A set of four-letter codes used in ecological/geographic regions in oceanography.
FIFA country code	?	sport/football	global	?	free	polygon	Administrative divisions.
FIPS country codes	1994?	scope	U.S.	?	free	polygon	Administrative divisions. (FIPS 10-4) area code.
FIPS place codes	?	U.S.	place	?	free	polygon	(FIPS 55). Administrative divisions.
FIPS country codes	?	U.S.	globe/nations	?	free	polygon	(FIPS 6-4). Administrative divisions
FIPS state codes	?	U.S.	?	?	free	polygon	(FIPS 5-2). Administrative divisions

Historical or less widely used

Geocode	Inception	Scope	Coverage	Formation	Ownership	Rep. entity	Context and description
HASC	?	general	nations and subdivs.	Name abbreviation	free	polygon	Administrative divisions. HASC stands "Hierarchical Administrative Subdivision Codes".
UTM Zone	?	general	?	?	free	grid cell	?
UTM Grid Zones	?	general	?	?	free	grid cell	based on UTM Zones, and Latitude bands ofMGRS..
WMO squares	~2005?	Meteorology	globe	grid	free	grid cell	... replaced by modern DGGS's...
C-squares	2002	general	globe	?	free	grid cell	compact encoding of geographic coordinate bounds (latitude-longitude). UsesWMO squaresas starting point for hierarchical subdivision.
GEOREF	?	general	?	?	free	polygon	World Geographic Reference System, a military / air navigation coordinate system for point and area identification
GARS	~2007?	general	?	?	free	polygon	reference system developed by the National Geospatial-Intelligence Agency (NGA)
MGRS	~1960s	general	?	?	free	grid cell	Military Grid Reference System. Derived from UTM and UPS grids by NATO with a unique naming convention.

Other examples

Other geocodes:

• S2:a geocoding scheme usingspherical geometryand the space-fillingHilbert curve,developed atGoogle
• H3:Hexagonal Hierarchical Spatial Index a geocoding scheme initially developed at Ubersource code availableand documented at h3geo
• Munich Orientation Convention:converts lat/lon to metrical monopolar codes for targets, crossings, stations, stop points, bridges, tunnels, towns, islands, volcanoes, highway exits etc.
• SALB(Second Administrative Level Boundaries), by UN
• OpenPostcode,opensource global algorithm (local adaptations as Irish & Hong Kong postcodes).
• WOEID
• OpenStreetMapshortlink, used as a short permanent link to map locations
• Quarter Degree Grid Cells
• NAC(patended), area codes (area can be indefinitely small)
• GEOID, the name ofUnited States Census Bureaugeographic identifiers.
• In the United States, the American 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 Systemin Canada

Other standards

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.

See also

• 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

External links

• Media related toGeocodesat Wikimedia Commons