Addresses are not fit to be displayed in a map context due to the fact that they lack a computer readable spatial reference. Geocoding allows the derivation of precise coordinates on the surface of the earth by assigning x-coordinates and y-coordinates to point locations. Geocoding is often the responsibility of a central mapping agency (Clarke, 1997).
Geocoding enhances data so the inherent information contained within addresses can be harnessed by the researcher, planner, developer, or analyst. By placing data points in a spatial context, the database can now be used within the powerful context of GIS, thereby increasing the functionality of the database itself.
Additional benefits of geocoding:
The spatial level of geocoding depends upon the intended uses of the data. Databases used for geocoding need to contain varying levels of information depending on the level of geocoding being completed. For example, to geocode at the zip code level, the database requires only one field, zip code. The most information is required for geocoding street addresses. Therefore, statistical analysis with map units equal or larger than zip codes are less demanding on address quality than routing applications that, for example, are intended to direct emergency vehicles to a location.
The geocoding process utilizes at least two data sets: one containing the address information without map position information and the other containing a reference street map or other address diction with known map position information. Other data sources (e.g. USPS data sets) may provide useful information to increase the match rate between address data set and street data set. A software package links records in both databases, by matching street names and addresses. Successful matches result in the addition of the map information, usually in latitude and longitude coordinates, to the original address database.
Potential Errors
Type I
Occurs when addresses have not been geocoded although the
address is valid. This is the most common error and is caused
by incomplete street data or a failure of the geocoder to
parse the address correctly.
Type II
Occurs when addresses are matched to the wrong street segment
or zip code. This error may be caused by bad or incomplete
address information or by incorrect parsing and cleaning of
the address by the geocoder. Geocoders attempt to correct
irregularities in two basic approaches. Some geocoders clean
an address by using third party data sets, such as the USPS
product suite. Another approach is to change or auto-complete
parts of the address based on 'best guesses'.
Some corrections of address irregularities may actually cause Type II errors. A typical example would be that the street prefix 'N' has been replaced by 'S' because the geocoder could not find the address on the north side of town, assumes a user input error and may, incorrectly, find the same street on the south side of town. Geocoders deal with irregularities differently resulting in different degrees of reliability of their results. Usually, geocoders indicate the level of correction employed to locate the address on a street network through an exception code.
Geocoding software produces different success levels. This may be due to additional data sets that are included with some geocoders or the level of effort that has been put into the design of the geocoders. Valid addresses should perform well under most geocoders. However, high-end geocoders may show a better resilience to address data irregularities.
Improving address quality in database may be the single most important step in targeting the challenges of the spatially enabled environment of modern organizations.
The In-Line Geocoder allows for the automatic geocoding of an address record, within a database, as soon as it is entered. The In-Line Geocoder can geocode single records or a batch of records.
The following list summarizes typical features of In-Line Geocoders:
Most commercial geocoding services require that their street data sources be obtained and used exclusively. Additionally, commercial geocoders often provide a matchcode in percentage. STC's Inline Geocoder adds exception codes to the result set that tell any user what happened to that address record. The exception codes can be queried and later re-geocoded when new street data become available.
Conversely, the In-Line geocoder allows consistent data at all times and near "real-time" mapping possibilities thus minimizing required man-hours involved to none. Therefore, the In-Line geocoder pays for itself.