Geocode

A geocode is a code that represents a geographic entity (location or object). In general the geocode is a human-readable and short unique identifier of the entity.

Typical geocodes and entities represented by it:

• Country code and subdivision code. Polygon of the administrative boundaries of a country or a subdivision: ISO 3166-1 alpha-2 code (e.g. AF for Afghanistan, BR for Brazil), ISO 3166-2:AF code (e.g. AF-GHO for Ghor province) or ISO 3166-2:BR code (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 6vd23gq at the Brazil's centroid) or a OLC code (e.g. ~0.004 km² cell 58Q8XXXX+X at the same point).

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

Geocodes are mainlly used for labelling, data integrity, geotagging and spatial indexing.

Classification

Geocode cells with 8 and 9 (yellow) digits, of a typical hierarchical grid, Geohash, comparing with latitude-longitude (12 or more digits). A museum is a typical location to use a geocode, pointing its gate need ~20 meters of precision.

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

• onwership: 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).

• 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 terrestral areas) or by the onwership'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).
  • grids can be also classified as hierarchical or non-hierarchical
  • special hierarchical grids, with global covering and equal-area cells, can be classified as DGGS cell
  • 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).

Geocode system

The set of all geocodes used as unique identifiers, in a well-defined context, is a geocode system (or 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: contry codes use two letters of the alphabet (chacacter set A-Z). The most commom way to describe formally is by regular expression (e.g. /[A-Z]{2,2}/).

• 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, 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 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 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.

Cataloged examples

This section lists most of the geocodes cataloged on Wikipedia, and shows a summarized description of each, based on the classification section.

In use, general scope

Geocodes in use and with general scope:

Geocode	Inception	Coverage	Formation	Onwership	Rep. entity	Context and description
ISO 3166 (alpha-2 and alpha-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	Hash notation for locations. See also Geohash-36.
Open Location Code (OLC)	2014	globe	encode(latLon,precision)	free	grid cell	See also PlusCodes.
What3words	2013	globe	encode(latLon)	patented	grid cell	patent-restrictions system, converts 3x3 meter squares into 3 words
Mapcode	2001	globe	encode(latLon)	patented	point	A mapcode is a code consisting of two groups of letters and digits, separated by a dot.

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:

Geocode	Inception	Coverage	Formation	Onwership	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?)	...
Local OLC (Cape Verde)	2016	postal regions	encode(latLon,precision)	free	grid cell	OLC is used to provide postal services.

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	Onwership	Rep. entity	Context and description
ONS code	2001	UK only	UK/themes	Serial number	free	polygon	Administrative divisions. Geographical areas of the UK, for use in tabulating census.
NUTS area code	2003	EU only	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	?	Canada only	?	Serial number	free	polygon	Administrative divisions, numeric codes. ... Statistical, like ONS.
UN/LOCODE	?	trade and transport	globe	Serial number	free	polygon	Administrative divisions. UN codes 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 to ISO 3166-1 alpha-2, see Country code top-level domain, List and Internationalized country codes.
IOC country codes	~1960	Sport	globe	abbreviation	free	polygon	Administrative divisions. Codes of IOC members; 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

Old, abandoned or less used

Geocode	Inception	Scope	Coverage	Formation	Onwership	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	polygon	?
UTM Grid Zones	?	general	?	?	free	polygon	based on UTM Zones, and Latitude bands of MGRS..
WMO squares	~2005?	Meteorology	globe	grid	free	polygon	... replaced by modern DGGS's ...
C-squares	?	general	?	?	free	polygon	compact encoding of geographic coordinate bounds (latitude-longitude). Use WMO squares.
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

• S2: a geocoding scheme using spherical geometry and the space-filling Hilbert curve, developed at Google

• 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 [1]

• OpenPostcode, opensource global algorithm (local adaptations as Irish & Hong Kong postcodes).

• WOEID
• OpenStreetMap shortlink, 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 of United States Census Bureau geographic identifiers.

See also

• Geocoding
• Geolocation
• Geotagging
• Geographic information retrieval
• Toponym Resolution