Centerline Magic
Introduction
For many years now the intelligent street centerline file (or the address range guide) has been one
of the most utilitarian tools in a Geographic Information System (GIS). The benefits of a street
centerline file go well beyond creating a nice looking street map. Street centerline files provide
quick references for address location, incident reporting and aggregation, gee-coding, vehicle
routing, and/or address matching. They are found in planning offices, utility companies and
emergency dispatch. One of the reasons these files are so versatile and useful is the fact that the
design of this tool is simple, practical and has withstood the test of time.
Although using this tool is simple, building a file is no easy matter. Many communities have
constructed digital versions of their centerline files, but have not propagated the address ranges
because of time and cost constraints. The process described in the following text provides a way
to efficiently and easily construct address ranges for a street centerline. It avoids costly field
collection and provides a method to audit the address ranges for files that were built using
traditional means.
Basic Street Centerline Model
The basic model for an intelligent street centerline file consists of a graphic and descriptive
tabular data set. Generally, each record, which is a graphic and tabular pair, in the data file
corresponds to a single street segment between intersecting streets. For example the segment of
Main St. between 1't Ave. and 2nd Ave. would be represented by one record in the centerline file.
The tabular data record is comprised of several descriptive fields. Some of the data fields
describe the segment as a whole entity and other fields describe characteristics lying to either the
left or right side of the street segment. Typically the fields that describe the entity as a whole are
the street segment's direction, name and type (such as, N - Main - St.). The data fields that
describe the left side and right characteristics are generally the left side address range and the
right side address range. The lell side / right side address information is often listed as a range
consisting of the low and high addresses.
The best way to describe a centerline address record is by using a real world example. Suppose
someone were compiling data for Main Street between 1't and 2nd Avenue. If the compiler
"stood" at the intersection of 1't and Main and faced 2nd Ave., he would look to his Iefi and
record the left address nearest the intersection in the left low address field. He would similarly
collect the address number of the structure on his right in the right low address data field. The
compiler would then proceed down Main Street. At 2nd Ave. he would look to his left, determine
the house number of the last structure nearest the intersection and record it in left high address
field. Then looking to his right he would likewise record the number for the right high address
field. The resulting record would look something like this:
Table 1: Standard Street Segment Address Format
| Record Id |
Dir. |
Name |
Type |
Lft_Low |
Lft-High |
Rgt-Low |
Rgt-High |
| 132 |
N |
Main |
St |
100 |
136 |
101 |
145 |
The record in the example above is based on a couple of assumptions. The first assumption is
that the graphic record (the coordinate record) and the tabular record both have "directionality."
The graphic record consists of a starting coordinate (or x,y pair) and an ending coordinate. Once
again referring to the example above, the starting coordinate (commonly referred to as the "from
node") for the segment on Main Street would be the coordinate pair at 1't and Main. R then
follows that the coordinates at the 2nd and Main Street intersection would be recorded as the
ending coordinate or "to node."
The second assumption deals with how the addresses are recorded. There are two schools of
thought regarding the recording of address ranges. In the example presented above, the recorded
address ranges represent the "actual" or "field verified" address ranges. The other possibility is
to record the "theoretical" or "hundred block" range. The hundred block approach records
addresses as complete one-hundred block ranges. Below is a comparison between the two
approaches.
Table 2: Actual and Theoretical Range Comparison
Actual
| Dir. |
Name |
Type |
Lft_Low |
Lft-High |
Rgt-Low |
Rgt-High |
| N |
Main |
St |
100 |
136 |
101 |
145 |
Theoretical
| Dir. |
Name |
Type |
Lft_Low |
Lft-High |
Rgt-Low |
Rgt-High |
| N |
Main |
St |
100 |
198 |
101 |
199 |
