Geocoding
Encyclopedia
Geocoding is the process of finding associated geographic coordinates (often expressed as latitude
and longitude
) from other geographic data, such as street addresses, or zip codes (postal codes). With geographic coordinates the features can be mapped and entered into Geographic Information Systems
, or the coordinates can be embedded into media such as digital photographs via geotagging
.
Reverse geocoding
is the opposite: finding an associated textual location such as a street address, from geographic coordinates.
A geocoder is a piece of software or a (web) service that helps in this process.
. This method makes use of data from a street geographic information system
where the street network is already mapped within the geographic coordinate space. Each street segment is attributed with address ranges (e.g. house numbers from one segment to the next). Geocoding takes an address, matches it to a street and specific segment (such as a block
, in towns that use the "block" convention). Geocoding then interpolates the position of the address, within the range along the segment.
Let's say that this segment (for instance, a block) of Evergreen Terrace runs from 700 to 799. Even-numbered addresses fall on the east side of Evergreen Terrace, with odd-numbered addresses on the west side of the street. 742 Evergreen Terrace would (probably) be located slightly less than halfway up the block, on the east side of the street. A point would be mapped at that location along the street, perhaps offset some distance to the east of the street centerline.
Difficulties arise when
While there might be 742 Evergreen Terrace in Springfield, there might also be a 742 Evergreen Terrace in Shelbyville. Asking for the city name (and state, province, country, etc. as needed) can solve this problem. Some situations require use of postal code
s or district name for disambiguation. For example, there are multiple 100 Washington Streets in Boston
, Massachusetts
because several cities have been annexed without changing street names.
Finally, several caveats on using interpolation:
A very common error is to believe the accuracy ratings of a given map's geocodable attributes. Such "accuracy" currently touted by most vendors has no bearing on an address being attributed to the correct segment, being attributed to the correct "side" of the segment, nor resulting in an accurate position along that correct segment. With the geocoding process used for U.S. Census
TIGER datasets, 5-7.5% of the addresses may be allocated to a different census tract
, while 50% of the geocoded points might be located to a different property parcel.
The accuracy of geocoded data can also have a bearing on the quality of research that can be done using this data. One study by a group of Iowa researcher's found that the common method of geocoding using TIGER datasets as described above, can cause a loss of as much as 40% of the power of a statistical analysis.An alternative is to use orthophoto or image coded data such as the Address Point data from Ordnance Survey in the UK, but such datasets are generally expensive.
Because of this, it is quite important to avoid using interpolated results except for non-critical applications, such as pizza delivery
. Interpolated geocoding is usually not appropriate for making authoritative decisions, for example if life safety will be affected by that decision. Emergency services, for example, do not make an authoritative decision based on their interpolations; an ambulance or fire truck will always be dispatched regardless of what the map says.
to aid in emergency response, maintenance, and navigation. It is also possible to use a combination of these geocoding techniques - using a particular technique for certain cases and situations and other techniques for other cases.
Geocoding is common on the web, for services like finding driving directions to or from some address, or finding a list of the geographically nearest store or service locations.
Geocoding is one of several methods of obtaining geographic coordinates for geotagging
media, such as photographs or RSS
items.
to the public, they also place disclaimers regarding the locational accuracy of points on the map, acknowledging these location masking techniques, and impose terms of use for the information.
Articles:
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
and longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
) from other geographic data, such as street addresses, or zip codes (postal codes). With geographic coordinates the features can be mapped and entered into Geographic Information Systems
Geographic Information System
A geographic information system, geographical information science, or geospatial information studies is a system designed to capture, store, manipulate, analyze, manage, and present all types of geographically referenced data...
, or the coordinates can be embedded into media such as digital photographs via geotagging
GeoTagging
Geotagging is the process of adding geographical identification metadata to various media such as a geotagged photograph or video, websites, SMS messages, QR Codes or RSS feeds and is a form of geospatial metadata...
.
Reverse geocoding
Reverse geocoding
Reverse geocoding is the process of back coding of a point location to a readable address or place name. This permits the identification of nearby street addresses, places, and/or areal subdivisions such as neighbourhoods, county, state, or country...
is the opposite: finding an associated textual location such as a street address, from geographic coordinates.
A geocoder is a piece of software or a (web) service that helps in this process.
Address interpolation
A simple method of geocoding is address interpolationInterpolation
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
. This method makes use of data from a street geographic information system
Geographic Information System
A geographic information system, geographical information science, or geospatial information studies is a system designed to capture, store, manipulate, analyze, manage, and present all types of geographically referenced data...
where the street network is already mapped within the geographic coordinate space. Each street segment is attributed with address ranges (e.g. house numbers from one segment to the next). Geocoding takes an address, matches it to a street and specific segment (such as a block
City block
A city block, urban block or simply block is a central element of urban planning and urban design. A city block is the smallest area that is surrounded by streets. City blocks are the space for buildings within the street pattern of a city, they form the basic unit of a city's urban fabric...
, in towns that use the "block" convention). Geocoding then interpolates the position of the address, within the range along the segment.
Example
Take for example: 742 Evergreen Terrace742 Evergreen Terrace
742 Evergreen Terrace is the fictional street address in Springfield of the Simpson family home in the animated television series, The Simpsons and in the feature film The Simpsons Movie. In the series the house is currently owned by Homer Simpson...
Let's say that this segment (for instance, a block) of Evergreen Terrace runs from 700 to 799. Even-numbered addresses fall on the east side of Evergreen Terrace, with odd-numbered addresses on the west side of the street. 742 Evergreen Terrace would (probably) be located slightly less than halfway up the block, on the east side of the street. A point would be mapped at that location along the street, perhaps offset some distance to the east of the street centerline.
Complicating factors
However, this process is not always as straightforward as in this example.Difficulties arise when
- distinguishing between ambiguous addresses such as 742 Evergreen Terrace and 742 W Evergreen Terrace.
- attempting to geocode new addresses for a street that is not yet added to the geographic information system database.
While there might be 742 Evergreen Terrace in Springfield, there might also be a 742 Evergreen Terrace in Shelbyville. Asking for the city name (and state, province, country, etc. as needed) can solve this problem. Some situations require use of postal code
Postal code
A postal code is a series of letters and/or digits appended to a postal address for the purpose of sorting mail. Once postal codes were introduced, other applications became possible.In February 2005, 117 of the 190 member countries of the Universal Postal Union had postal code systems...
s or district name for disambiguation. For example, there are multiple 100 Washington Streets in Boston
Boston
Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
because several cities have been annexed without changing street names.
Finally, several caveats on using interpolation:
- The typical attribution of a street segment assumes that all "even" numbered parcels are on one side of the segment, and all "odd" numbered parcels are on the other. This is often not true in real life.
- Interpolation assumes that the given parcels are evenly distributed along the length of the segment. This is almost never true in real life; it is not uncommon for a geocoded address to be off by several thousand feet.
- Segment Information (esp. from sources such as TIGERTigerThe tiger is the largest cat species, reaching a total body length of up to and weighing up to . Their most recognizable feature is a pattern of dark vertical stripes on reddish-orange fur with lighter underparts...
) includes a maximum upper bound for addresses and is interpolated as though the full address range is used. For example, a segment (block) might have a listed range of 100-199, but the last address at the end of the block is 110. In this case, address 110 would be geocoded to 10% of the distance down the segment rather than near the end. - Most interpolation implementations will produce a point as their resulting "address" location. In reality, the physical address is distributed along the length of the segment, i.e. consider geocoding the address of a shopping mallShopping mallA shopping mall, shopping centre, shopping arcade, shopping precinct or simply mall is one or more buildings forming a complex of shops representing merchandisers, with interconnecting walkways enabling visitors to easily walk from unit to unit, along with a parking area — a modern, indoor version...
- the physical lot may run quite some distance along the street segment (or could be thought of as a two-dimensional space-filling polygon which may front on several different streets - or worse, for cities with multi-level streets, a three-dimensional shape that meets different streets at several different levels) but the interpolation treats it as a singularity.
A very common error is to believe the accuracy ratings of a given map's geocodable attributes. Such "accuracy" currently touted by most vendors has no bearing on an address being attributed to the correct segment, being attributed to the correct "side" of the segment, nor resulting in an accurate position along that correct segment. With the geocoding process used for U.S. Census
United States Census
The United States Census is a decennial census mandated by the United States Constitution. The population is enumerated every 10 years and the results are used to allocate Congressional seats , electoral votes, and government program funding. The United States Census Bureau The United States Census...
TIGER datasets, 5-7.5% of the addresses may be allocated to a different census tract
Census tract
A census tract, census area, or census district is a geographic region defined for the purpose of taking a census. Usually these coincide with the limits of cities, towns or other administrative areas and several tracts commonly exist within a county...
, while 50% of the geocoded points might be located to a different property parcel.
The accuracy of geocoded data can also have a bearing on the quality of research that can be done using this data. One study by a group of Iowa researcher's found that the common method of geocoding using TIGER datasets as described above, can cause a loss of as much as 40% of the power of a statistical analysis.An alternative is to use orthophoto or image coded data such as the Address Point data from Ordnance Survey in the UK, but such datasets are generally expensive.
Because of this, it is quite important to avoid using interpolated results except for non-critical applications, such as pizza delivery
Pizza delivery
Pizza delivery is a service in which a pizzeria delivers a pizza to a customer. Delivery is normally made with an automobile, motor scooter, or bicycle.- Ordering :...
. Interpolated geocoding is usually not appropriate for making authoritative decisions, for example if life safety will be affected by that decision. Emergency services, for example, do not make an authoritative decision based on their interpolations; an ambulance or fire truck will always be dispatched regardless of what the map says.
Other techniques
Other means of geocoding might include locating a point at the centroid (center) of a land parcel, if parcel (property) data is available in the geographic information system database. In rural areas or other places lacking high quality street network data and addressing, GPS is useful for mapping a location. For traffic accidents, geocoding to a street intersection or midpoint along a street centerline is a suitable technique. Most highways in developed countries have mile markersMilestone
A milestone is one of a series of numbered markers placed along a road or boundary at intervals of one mile or occasionally, parts of a mile. They are typically located at the side of the road or in a median. They are alternatively known as mile markers, mileposts or mile posts...
to aid in emergency response, maintenance, and navigation. It is also possible to use a combination of these geocoding techniques - using a particular technique for certain cases and situations and other techniques for other cases.
Research
Recent research has introduced a new approach to the control and knowledge aspects of geocoding, by using an agent-based paradigm. In addition to the new paradigm for geocoding, additional correction techniques and control algorithms have been developed. The approach represents the geographic elements commonly found in addresses as individual agents. This provides a commonality and duality to control and geographic representation. In addition to scientific publication, the new approach and subsequent prototype gained national media coverage in Australia. The research was conducted at Curtin University in Perth, Western Australia.Uses
Geocoded locations are useful in many GIS analysis and cartography tasks.Geocoding is common on the web, for services like finding driving directions to or from some address, or finding a list of the geographically nearest store or service locations.
Geocoding is one of several methods of obtaining geographic coordinates for geotagging
GeoTagging
Geotagging is the process of adding geographical identification metadata to various media such as a geotagged photograph or video, websites, SMS messages, QR Codes or RSS feeds and is a form of geospatial metadata...
media, such as photographs or RSS
RSS
-Mathematics:* Root-sum-square, the square root of the sum of the squares of the elements of a data set* Residual sum of squares in statistics-Technology:* RSS , "Really Simple Syndication" or "Rich Site Summary", a family of web feed formats...
items.
Privacy concerns
The proliferation and ease of access to geocoding (and reverse-geocoding) services raises privacy concerns. For example, in mapping crime incidents, law enforcement agencies aim to balance the privacy rights of victims and offenders, with the public's right to know. Law enforcement agencies have experimented with alternative geocoding techniques that allow them to mask some of the locational detail (e.g., address specifics that would lead to identifying a victim or offender). As well, in providing online crime mappingCrime mapping
Crime mapping is used by analysts in law enforcement agencies to map, visualize, and analyze crime incident patterns. It is a key component of crime analysis and the CompStat policing strategy...
to the public, they also place disclaimers regarding the locational accuracy of points on the map, acknowledging these location masking techniques, and impose terms of use for the information.
List of some geocoding systems
Other systems (some of these code systems are free for use, others have different licences):- ISO 6709ISO 6709ISO 6709 Standard representation of geographic point location by coordinates is the international standard for representation of latitude, longitude and altitude for geographic point locations....
Standard Representation for Geographic Point Location by Coordinates - C-squaresC-squaresC-squares is a system of geocodes that provides a basis for simple spatial indexing of geographic features or data. It was devised by Tony Rees of CSIRO Marine and Atmospheric Research in 2001-2, and described in the literature in 2003...
- compact encoding of geographic coordinate bounds (latitude-longitude) - FIPS country codes (FIPS 10-4), area code, administrative, free
- FIPS place codes (FIPS 55) U.S.United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
only, free - FIPS county codeFIPS county codeThe FIPS county code is a five-digit Federal Information Processing Standard code which uniquely identifies counties and county equivalents in the United States, certain U.S. possessions, and certain freely associated states. The first two digits are the FIPS state code and the last three are...
s (FIPS 6-4) US only, free - FIPS state codeFIPS state codeFIPS state codes were numeric and two-letter alphabetic codes defined in U.S. Federal Information Processing Standard Publication 5-2 to identify U.S. states and certain other associated areas...
s (FIPS 5-2) US only, free - Canadian Location Code, encodes a weather forecast regionForecast regionA forecast region is a unique geographical area for which individual weather reports are issued.-Canada:In Canada, the Meteorological Service divides the country into several forecast regions for the purpose of issuing routine text forecasts and weather warnings, as Canada's provinces and...
- GeohashGeohashGeohash is a latitude/longitude geocode system inventedby Gustavo Niemeyer when writing the web service at geohash.org, andput into the public domain...
, compact string encoding of a geographic coordinate with arbitrary precision, in public domain - GeorefGeorefGeoref is a grid-based method of specifying locations on the surface of the Earth. Georef is essentially based on the geographic system of latitude and longitude, but using a simpler and more flexible notation. Georef was used primarily for air navigation, particularly in military or...
, a military / air naviation coordinate system for point and area identification - HASCHierarchical administrative subdivision codesHierarchical administrative subdivision codes are codes to represent names of country subdivisions, such as states, province, regions.They are described in the book "Administrative Subdivisions of Countries: A Comprehensive World Reference, 1900 Through 1998" written by Gwillim Law.The codes are...
(Hierarchical administrative subdivision codes) - IATA airport codeIATA airport codeAn IATA airport code, also known an IATA location identifier, IATA station code or simply a location identifier, is a three-letter code designating many airports around the world, defined by the International Air Transport Association...
s, area /point codes, airports - ICAO airport codeICAO airport codeThe ICAO airport code or location indicator is a four-character alphanumeric code designating each airport around the world. These codes are defined by the International Civil Aviation Organization, and published in ICAO Document 7910: Location Indicators.The ICAO codes are used by air traffic...
s, area /point codes, airports - IANA country codes similar to ISO 3166-1 alpha-2ISO 3166-1 alpha-2ISO 3166-1 alpha-2 codes are two-letter country codes defined in ISO 3166-1, part of the ISO 3166 standard published by the International Organization for Standardization , to represent countries, dependent territories, and special areas of geographical interest...
- IOC country codes, area, worldwide
- ISO 3166ISO 3166ISO 3166 is a standard published by the International Organization for Standardization . It defines codes for the names of countries, dependent territories, special areas of geographical interest, and their principal subdivisions . The official name of the standard is Codes for the representation...
country and subdivision codes - ITU-R country codeITU letter codesThe radiocommunication division of the International Telecommunication Union uses the following letter codes to identify its member countries. Eight countries are assigned single-letter codes, while the rest have codes three letters in length...
s - ITU-T country calling codes
- ITU-T mobile calling codes
- Maidenhead Locator SystemMaidenhead Locator SystemThe Maidenhead Locator System is a geographic coordinate system used by amateur radio operators. Dr. John Morris, G4ANB, originally devised the system, and a group of VHF managers, meeting in Maidenhead, England in 1980, adopted it...
- MapDot Protocol: world locations coded into a zone sequence http://www.mapdot.info/, free
- MARC country codes
- Marsden SquareMarsden squareMarsden square mapping or Marsden squares is a system that divides a chart of the world with latitude-longitude gridlines between 80°N and 70°S latitudes into grid cells of 10° latitude by 10° longitude, each with a unique, numeric identifier...
s - NACNatural Area CodeThe Natural Area Code is a proprietary geocode system for identifying an area anywhere on the Earth, or a volume of space anywhere around the Earth...
, area codes (area can be indefinitely small) - NUTSNomenclature of Territorial Units for StatisticsThe Nomenclature of Territorial Units for Statistics or Nomenclature of Units for Territorial Statistics is a geocode standard for referencing the subdivisions of countries for statistical purposes...
area code, partially administrative, worldwide: countries, Europe : country to community - ONSONS coding systemIn the United Kingdom, the Office for National Statistics maintains a series of codes to represent a wide range of geographical areas of the UK, for use in tabulating census and other statistical data...
code, UKUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
only, administrative - Postal codePostal codeA postal code is a series of letters and/or digits appended to a postal address for the purpose of sorting mail. Once postal codes were introduced, other applications became possible.In February 2005, 117 of the 190 member countries of the Universal Postal Union had postal code systems...
s, area, worldwide, country-codes by UPUUpuUpu, also called Apu , was the region surrounding Damascus of the 1350 BC Amarna letters. Damascus was named Dimašqu/Dimasqu/ etc. Upu, also called Apu (and Ubi or Upi by some authors), was the region surrounding Damascus of the 1350 BC Amarna letters. Damascus was named Dimašqu/Dimasqu/ etc. Upu,...
, free - Quarter Degree Grid CellsQDGCQDGC - Quarter Degree Grid Cells are a way of dividing the longitude latitude degree square cells into smaller squares, forming in effect a system of geocodes. Historically QDGC has been used in a lot of African atlases. Several African biodiversity projects uses QDGC, among which The atlas of...
- UN M.49UN M.49UN M.49 is a standard for area codes used by the United Nations for statistical purposes, developed and maintained by the United Nations Statistics Division...
region codes, area code, continents, countries (like ISO 3166-1 numericISO 3166-1 numericISO 3166-1 numeric codes are three-digit country codes defined in ISO 3166-1, part of the ISO 3166 standard published by the International Organization for Standardization , to represent countries, dependent territories, and special areas of geographical interest...
) - SALB (Second Administrative Level Boundaries), by UN http://www.unsalb.org/
- SGC codesStandard Geographical Classification code (Canada)The Standard Geographical Classification is a system maintained by Statistics Canada for categorizing and enumerating the census geographic units of Canada. Each geographic area receives a unique numeric code ranging from 1 to 10 digits, which extend telescopically to refer to increasingly small...
, CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
only, statistical - UN/LOCODEUN/LOCODEUN/LOCODE, the United Nations Code for Trade and Transport Locations, is a geographic coding scheme developed and maintained by United Nations Economic Commission for Europe , a unit of the United Nations. UN/LOCODE assigns codes to locations used in trade and transport with functions such as...
, area, administrative, cities - UTM
- WMO squaresWorld Meteorological Organization squaresWorld Meteorological Organization squares or WMO squares is a system of geocodes that divides a chart of the world with latitude-longitude gridlines into grid cells of 10° latitude by 10° longitude, each with a unique, 4-digit numeric identifier...
Republic of Ireland Geocodes
The Republic of Ireland doesn't yet have postcodes, unlike nearly every other country in the world, however a number of independent geolocation based codes are available.- Loc8code, eight character licensed code, accurate to +/-6m.
- GoCode, seven character licensed code accurate to +/-5m.
- OpenPostcode, opensource, a seven digit code accurate to +/- 3m, plus a checksum digit.
See also
- GeotaggingGeoTaggingGeotagging is the process of adding geographical identification metadata to various media such as a geotagged photograph or video, websites, SMS messages, QR Codes or RSS feeds and is a form of geospatial metadata...
- GeolocationGeolocationGeolocation is the identification of the real-world geographic location of an object, such as a radar, mobile phone or an Internet-connected computer terminal...
- Geocoded photoGeocoded photoA geotagged photograph is a photograph which is associated with a geographical location by geotagging. Usually this is done by assigning at least a latitude and longitude to the image, and optionally altitude, compass bearing and other fields may also be included.In theory, every part of a picture...
, which includes methods of geocoding images - C-squaresC-squaresC-squares is a system of geocodes that provides a basis for simple spatial indexing of geographic features or data. It was devised by Tony Rees of CSIRO Marine and Atmospheric Research in 2001-2, and described in the literature in 2003...
and QDGCQDGCQDGC - Quarter Degree Grid Cells are a way of dividing the longitude latitude degree square cells into smaller squares, forming in effect a system of geocodes. Historically QDGC has been used in a lot of African atlases. Several African biodiversity projects uses QDGC, among which The atlas of...
, geocoding methods for statistical mapping (marine life, avian populations, humans, etc.) - GeoreferenceGeoreferenceTo georeference something means to define its existence in physical space. That is, establishing its location in terms of map projections or coordinate systems. The term is used both when establishing the relation between raster or vector images and coordinates, and when determining the spatial...
- Reverse geocodingReverse geocodingReverse geocoding is the process of back coding of a point location to a readable address or place name. This permits the identification of nearby street addresses, places, and/or areal subdivisions such as neighbourhoods, county, state, or country...
- BroadMapBroadMapBroadMap is a full-service GIS company based in Lebanon, New Hampshire. The company specializes in providing government, non-profit and commercial business partners with custom-fit, turnkey geospatial products and services.-Overview:...
- Free Global Latitude/Longitude Calculator - Free Interactive Geocoder For determination of Longitude and Latitude values - No advertisements.
External links
Books:- A Geocoding Best Practices Guide. Goldberg, Daniel W. (2008). Springfield, IL: NAACCR - Free online book describing the geocoding process and offering recommendations from the North American Association of Central Cancer RegistriesNorth American Association of Central Cancer Registries-North American Association of Central Cancer Registries, Inc. :Established in 1987, NAACCR, Inc. is a collaborative umbrella organization for cancer registries, governmental agencies, professional associations, and private groups in North America interested in enhancing the quality and use of...
(NAACCR)
Articles:
- Three Standard Geocoding Methods (in North America) - article
- The Evolution of Geocoding: Moving Away from Conflation Confliction to Best Match - article
- A Flexible Addressing System for Approximate Geocoding - paper presented at Geoinfo 2003
- The UCDP and AidData codebook on geo-referencing aid - guide for geocoding development aid projects