Geocoding

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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.

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[edit] 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.

[edit] 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 would fall on one side (e.g. west side) of Evergreen Terrace, with odd-numbered addresses on the other side (e.g. east side). 742 Evergreen Terrace would (probably) be located slightly less than halfway up the block, on the west side of the street. A point would be mapped at that location along the street, perhaps offset some distance to the west of the street centerline.

[edit] 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. Some situations require use of postal codes or district name for disambiguation. For example, there are multiple 100 Washington Streets in Boston, Massachusetts[1] 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. [2]

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 impacted 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.

[edit] 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 markers to aid in emergency response, maintenance, and navigation.

Address locators that use different reference data types or that require different input information can be combined in a Composite Address Locator. An address is processed through each locator in turn, until a match is found. This method can be helpful if the input address table may have inaccurate addresses or incomplete address information. A common use of a composite address locator is combining a street address locator with a zip code locator. When an address is not matched according to its street address, the zip code can be used to associate this address with a zip code region center point.

An alternate name table can be used when a single street segment may have multiple names. For example, the main road through the center of Saratoga Springs, NY is called Broadway, but portions of this road are also associated with State Highway 50, Route 29, Route 9, and Route 9N. In order to account for addresses that use any of these street names, an alternate name table can be used to check for the various names.

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.

[edit] 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 media, such as photographs or RSS items.

[edit] Unique Feature Identifier

A Unique Feature Identifier (UFI) is a number used for geocoding that uniquely identifies cities, towns, villages and other geographic features.[3] For example, the Rio Paya (a stream) in Venezuela has the Unique Feature Identifier -955094.[4]

[edit] 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 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.

[edit] List of some geocoding systems

Some of these code systems are free for use, others have different licences.

[edit] List of Geocoding Web Services

[edit] See also

[edit] 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". International Journal of Geographic Information Sciences 15 (5). http://jratcliffe.net/papers/Ratcliffe%20(2001)%20On%20the%20accuracy%20of%20TIGER-type%20geocoding.pdf. 
  3. ↑ GNS Help. National Geospatial Intelligence Agency Web site.Accessed 17 September 2010.
  4. ↑ Geographical name data for Rio Paya, Venezuela. Geographic.org Web site.

[edit] External links

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