Geocoding

Not to be confused with Geotagging.

Geocoding is the process of transforming a description of a location (such as an address, name of a place, or coordinates) to a location on the earth's surface. Geocoding typically refers to the transformation process of addresses and places to coordinates, and is sometimes called forward geocoding whereas Reverse geocoding uses geographic coordinates to find a description of the location, most typically a postal address or place name.

The geographic coordinates used to represent locations vary greatly in positional accuracy. Examples include building centroids, land parcels, street addresses, centroids of postal codes (e.g. ZIP codes, CEDEX), and Administrative Boundary Centroids.

Geocoding is often trivialized as putting "points on a map" and this description is useful to those who are new to spatial processing. However the primary use for Geocoding to facilitate the GeoEnrichment process, that is to say, the process of using spatial analysis and spatial processing technologies (Enterprise Location Intelligence systems and Geographic Information Systems), to append the attributes from a spatially referenced file onto the address. For instance, to append the Flood Zone characteristics to a residential property to assess it for insurance risk purposes requires a geocode followed by a point-in-polygon.

Geocoding (verb) is the act of transforming gaspatial locationally descriptive text into a valid spatial representation.

A geocoder (noun) is a piece of software or a (web) service that implements a geocoding process i.e. a set of inter-related components in the form of operations, algorithms, and data sources that work together to produce a spatial representation for descriptive locational references.

A geocode (noun) is a spatial representation of a descriptive locational reference.

Address interpolation

A simple method of geocoding is address interpolation. 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.

Example

Take for example: 742 Evergreen Terrace

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 a 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

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. Boston, Massachusetts[1] has multiple "100 Washington Street" locations because several cities have been annexed without changing street names, thus requiring use of unique postal codes or district names for disambiguation. Geocoding accuracy can be greatly improved by first utilizing good address verification practices. Address verification will confirm the existence of the address and will eliminate ambiguities. Once the valid address is determined, it is very easy to geocode and determine the latitude/longitude coordinates. 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 a study of Australia's TIGER-like system found that 50% of the geocoded points were mapped to the wrong property parcel.[2] The accuracy of geocoded data can also have a bearing on the quality of research that can be done using this data. One study[3] by a group of Iowa researchers 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.

Point-level geocoding

Point-level geocoding offers a more accurate means of geocoding. This incorporates locating a point at the centroid (center) of a land parcel or of a building, thus improving the geocoding accuracy. This level of detail may not be critical for most applications, but for applications such as insurance underwriting, flood determination , and Call-Before-You-Dig, a few feet can make a very big difference.[4]

Other techniques

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 markers 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. In contrast to geocoding of structured postal address records, toponym resolution maps place names in unstructured document collections to their corresponding spatial footprints.

Research

Recent research has introduced a new approach to the control and knowledge aspects of geocoding, by using an agent-based paradigm.[5] In addition to the new paradigm for geocoding, additional correction techniques and control algorithms have been developed.[6] 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.[7] The research was conducted at Curtin University in Perth, Western Australia.[8]

Uses

Geocoded locations are useful in many GIS analysis, cartography, decision making workflow, transaction mash-up, or injected into larger business processes. On the web, geocoding is used in services like routing and local search. Geocoding, along with GPS provides location data for geotagging media, such as photographs or RSS 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 a portion of the locational detail (e.g., address specifics that would lead to identifying a victim or offender). As well, in providing online crime mapping 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.

See also

References

  1. ↑ Google Maps
  2. ↑ Ratcliffe, Jerry H. (2001). "On the accuracy of TIGER-type geocoded address data in relation to cadastral and census areal units" (PDF). International Journal of Geographic Information Sciences 15 (5).
  3. ↑ Mazumdar S, Rushton G, Smith B et al.. Geocoding accuracy and the recovery of relationships between environmental exposures and health. International Journal of Health Geographics. 2008;7:1–13. doi:10.1186/1476-072X-7-13. PMID 18387189.
  4. ↑ Point Level Geocoding in Group 1 Product
  5. ↑ Hutchinson, Matthew J (2010). Developing an Agent-Based Framework for Intelligent Geocoding (Ph.D. thesis). Curtin University.
  6. ↑ An Agent-Based Framework to Enable Intelligent Geocoding Services
  7. ↑ Jennifer Foreshew (24 November 2009). "Difficult addresses no problem for IntelliGeoLocator". The Australian. Retrieved 9 May 2011.
  8. ↑ Department of Education, Western Australia (April 2011). "X marks the spot". School Matters. Retrieved 9 May 2011.

External links

Books:

Articles:

Geocoding services:

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