geocoding

otheruses Geocoding is the process of assigning geographic coordinates (e.g. latitude-longitude) to street addresses, as well as other points and features. With geographic coordinates, the features can then be mapped and entered into Geographic Information Systems.

Address interpolation - For the United States and many other countries, an address interpolation method is commonly used. This method makes use of street Geographic Information SystemsGIS 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 city blockblock, 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 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 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.

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.

Geocoding new addresses for a street that is not yet added to the GIS database.While there might be 742 Evergreen Terrace in Springfield, there might also be a 742 Evergreen Terrace in Shelbyville. Using the postal code or other zone attribute, along with the address, allows geocoding to match an address (742 Evergreen Terrace) to a street in a particular town (Springfield), distinguishing it from all other 742 Evergreen Terrace addresses in other towns.Finally, it should be noted that there are several caveats that are often overlooked when 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.

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 mall - the physical lot may run quite some distance along the street segment, while interpolation treats it as a singularity.

A very common error is to believe the accuracy ratings of a given map's geocodable attributes. Taking one specific vendor as an example, who claims a "99% match" in selected areas - the "match" simply means that the address was found in the attributes. It does *not* mean the address is attributed to the correct segment, or that the attribution of that segment will interpolate a correct result when this specific address is tried. In truth, the actual "wrong" count will be higher; if an address geocodes to a location that is off by 160 miles in the middle of the ocean (true case), it will still typically be counted as "correct" by the map vendor, since the address was found in the 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.

Because of this, it is quite important to avoid using interpolated results outside of anecdotal 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. (And as a side note, remember that 911 centers 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. "Making an authoritative decision" would be the case in which a cruise missile is directed at a longitude-latitude coordinate that is the result of an interpolated address. Nine times out of ten, the wrong house would be blown up. More common "authoritative decisions" would include deciding if a geocoded address does or does not lie on top of another feature, such as an overlay of a high-pressure gas main. If the interpolation were off by the width of the main's buffer, the interpolated address would not intersect the pipeline, and the wrong target would have been attacked.)

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 GIS 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. 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.

Data privacy concerns - In some situations, such as mapping the address of a crime incident, privacy concerns are raised as 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. 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 -

Geocoded photo

External links -

geocodeamerica.com - Geocode America : US Address Geocoding. Java/.NET/Ruby sample soap/rest apis. Free for non-commerical use

geocoder.us - Geocoder.us : Given an address in US, it returns latitude and longitude so that you can show the place on a map

geocoder.ca - Geocoder.ca: Given an address in Canada, it returns latitude and longitude so that you can show the place on a map

geocoder.com.ar - Geocoder.com.ar: Given an address in Argentina, it returns latitude and longitude so that you can show the place on a map

ontok.com - Ontok.com : Bulk geocoding in the US

earth-info.nga.mil - GEOnet Names Server: Freely available GIS information for areas outside of the U.S.A. and Antarctica, updated monthly by the National Geospatial-Intelligence Agency (NGA) and the U.S. Board on Geographic Names (US BGN)

geonames.usgs.gov - Geographic Names Information System (GNIS): freely available database containing information on almost 2 million physical and cultural geographic features in the United States and its !territoriesCategory:GeocodesCa tegory:Geographic? information system

GeoLytics - US Census, demographic, and GIS data products
GeoLytics is a leading provider of US Census, demographic, and GIS data products. GeoLytics' software products integrate US Census, population estimates, housing data, TIGER, and other data, with geographic boundaries and mapping software. Use for demographic research, marketing, and market research
http://www.tigermaps.com/

Mapquest
Provides online maps, driving directions and trip planning. The toolbar provides the map functionality in your browser.
http://www.mapquest.com

TravelGIS
Travel information on interactive maps and databases with free listings and immediate appearing on maps and listings.
http://www.travelgis.com/