GIS Cookbook: Background - Background Information about Geocoding 
Keywords: geoprocessing, geocode, geocoding, address, address matching, street, zip, zone, address number, street name, location, address table, Extended 911, geolocating, condominium, apartment, multi-unit complex, Japan addres
Category: Geocoding
Software: ArcView 3.2

Background: Geocoding in the simplest terms is "the act of assigning locations to things within the geographic frame of reference" (Goodchild, 2001). Geocoding has many synonyms including georeferencing and geolocating. The key to a georeferencing system is that each location has a unique code. This is possible only to a certain limit because of the overwhelming amount of information that would be needed for all things to have a unique code in the reference frame of the entire globe. Commonly, address number with street address and zip code are used to locate buildings within a reference frame. Street direction (i.e. north, east, south, or west) and street type (e.g. street, avenue, boulevard, etc.) may also be added to increase the precision of the output. Addresses are inputted in a tabular form and then matched with a geocoded reference theme. The reference theme is a digital map that has all the locations geocoded and can be displayed graphically or in tabular form. The reference theme includes many categories of information about each street location by listing street name, city name, zip code, the range of numbers on the right and left side of the street, street type, and street direction. Geocoding is not only limited to street addresses, for a less precise geocoding process, one can use only the zoning description, such as zip code or zoning number assigned by the city zoning ordinance. Different types of codes used for differentiating one location from another can be used to geocode within ArcGIS 8 by selecting a specific geocoding service.

Pros: There are many advantages to using the geocoding tool within a GIS. It can be applied in numerous social science fields and makes spatial analysis much more efficient. After a researcher has attained the necessary data to make a geocoded map (the database file of addresses and a geocoded reference theme), processing and producing the final geocoded address map may take as little as twenty minutes. After all the addresses have been mapped, one can do pattern analysis and many other types of spatial analysis.

The images above show a table of addresses (upper right) being geocoded using the reference theme (upper left) to create the final map on the bottom (center).

Cons: There a few disadvantages to geocoding with addresses. A drawback is the lack of uniqueness of each location as the area of interest gets bigger. Almost every city has a street called "Main Street." To avoid this problem one can assign more precise descriptors as mentioned above, such as direction or street type.

Many problems arise when working with rural areas. The street address numbers are given as a range of number on either side of a road and are assumed to be evenly spaced apart. This becomes very inaccurate in rural areas where the distances between houses vary considerably. Many rural addresses are just labeled as rural delivery route numbers, but sequential numbering on houses is increasingly common because of Extended "911", which mandates local governments to re-number their rural addresses in the interests of improving emergency response (Goodchild, 2002).

When geocoding addresses that are within high-density areas, one can experience problems similar to geocoding rural addresses. Structures such as condominiums, apartments, and other multi-unit complexes do not always have addresses ordered sequentially along roads. Also, within some apartment complexes of multiple buildings, some streets are not named, causing another confusion as to how to give each address a unique set of information.

When viewing the results of a geocoded list of addresses, one should be aware of the level of precision and accuracy of the end product. Geocoding gives the coordinates of the building as being directly lined up at the street whereas in real life, some structures may be set back very far from the road, such as if a house or business had a very long driveway.

Authored by: Sam Ying Modified: 9/10/02

Copyright © 2002-2015 by Regents of University of California, Santa Barbara
Cookbook: Ben Sprague, Ethan Sundilson, Carlin Wong, Sam Ying