Geocoding Technology A Useful Tool For Enterprise Applications
Robert W. Meador
Geographic Data Technology, Inc.
11 Lafayette StreetLebanon, NH 03766
What is geocoding?
Geocoding is the process of locating a street address on a map or assigning
geographic coordinates to that address. A "geocoder" - actually a software process -
standardizes each street address and assigns it a latitude and longitude based on an
Address Coding Guide or ACG. The ACG consists of street network data with address
ranges assigned to each block by the U.S. Postal Service. These ranges are frequently
assigned in groups of 100 potential addresses per block, and they have parity - that is,
odd-numbered addresses are on one side of the street and evens on the other.
Commercial data sets today generally have a "linkage" of around 85% nationwide.
This means that the Postal Service can provide address information for 85% of the
streets contained in an ACG. Postal information is updated monthly by the Postal
Service to add or change street names, address ranges, ZIP Codes and other detailed
information. Currency of the USPS information is a key issue in geocoding since most
users - utilities included - are very interested in the addresses for new homes and
businesses.
When given an address, the geocoder first tries to locate the street on which the
address falls. If it does find the street, the software interpolates the address location
along the street range. For example, 50 Main Street would be geocoded at the midpoint
of the block, and 99 Main Street would be geocoded at the end of the block.
If the street can't be found, the geocoder defaults to the "ZIP+4", which is a small,
specific delivery subdivision - such as one floor of an office building or one side of a
street block - within a five-digit ZIP Code. Addresses that default to a ZIP+4 are
geocoded at the midpoint of the block.
Failing to locate the appropriate ZIP+4, the geocoder will attempt to position the
address within a group of blocks known as a "ZIP+2" sector. At the largest geographic
level, the geocoder will position the address at the "centroid" (or balance point) of the
delivery addresses within a 5-digit ZIP Code. For most applications, particularly
demographic and marketing analysis, ZIP+2 and ZIP Code locations are close enough.
Geocoding software provide statistical information detailing how many addresses
were matched at which levels, for example, 75% at the street level, 17% at ZIP+4 level,
5% at ZIP+2, and 3% at ZIP Code level.
Geocoding vendors that offer interactive matching and manual matching services
can process addresses that don't geocode automatically. During the interactive phase,
map technicians assign additional geocodes by correcting street names and other
information from the provided file. For the manual process, digital map technicians call
to consult local authorities and use other resources to locate addresses and assign the
appropriate codes.
Block-face Accuracy
Some applications require better latitude and longitude information than the
interpolated point along a range of possible addresses. "Block-face accuracy" refers the
actual range of addresses on a particular block. Using the example above, lets assume
Main Street actually has the first house at 45 Main Street and the last house at 75 Main
Street. In this case the geocoded point would be interpolated between this range of 45
to 75 instead of the potential range provided by the Postal Service, giving a much more
precise latitude/longitude for the actual location. Although not perfect, block-face
accuracy eliminates the frequent problem of geocoded addresses bunching up at one
end of the street. Obviously, then, it is in the best interest of users to geocode with as
much block-face accurate data as possible. GDT is working with a number of utilities
and telecommunications companies to improve accuracy based on feedback from the
field.
By geocoding your customer information system (CIS) data and displaying it on a
map, you can get a quick indication of which streets have "bunches" of geocoded points
that are not likely to reflect the real world. Field technicians can then visit the site and
determine the correct address range for that street. By providing this address range
back to the vendor, you can receive the changed information in the next scheduled
update of the data set.
