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 inputted 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 GIS 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 end of the block to the next). Geocoding takes an address, matches it to a street and specific segment (block). Geocoding then interpolates the position of the address, within the range along the block.
Example
Take for example: 742 Evergreen Terrace
Let's say that this segment (block) of Evergreen Terrace runs from 700 to 799. Even-numbered addresses would fall on one side (e.g. left side) of Evergreen Terrace, with odd-numbered addresses on the other side (e.g. right side). 742 Evergreen Terrace would be located slightly less than halfway up the block, on the left side of the street. A point would be mapped at that ___location along the street, perhaps offset some distance to the left 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.
Also, while there might be 742 Evergreen Terrace in Springfield, there might also be a 742 Evergreen Terrace in Shelbyville. Using the zip 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.
Other techniques
Other means of geocoding might be to locate a point at the centroid (center) of a land parcel, if the 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. It is also possible to use a combination of geocoding techniques, depending on the particular situation or needs.
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.