Constructing Isolated Towns and Zip Code Information

Constructing Isolated Towns and Zip Code Information

Allan Collard-Wexler

Monday, March 20, 2006


Isolated Towns


I choose my markets to be the area around towns in the Continental United States. I get the data on places in the United States from the U.S. Census bureau.[1] However, to limit the issue of competitors in other towns affecting the pricing behavior in the central place, I need to find towns that are isolated, i.e. towns for which there is no other place located nearby.


First, I need to drop places in my dataset below a certain population threshold. In the Continental U.S. there are many very small towns, such asWestern Grove, Arizona which only had 415 inhabitants as of 1990. These small towns are unlikely to support most types of retail activity (such as the operation of a ready-mix concrete plant). Thus small towns should not be considered as potential sources of competition for establishments in larger towns. When I verify that any particular town is isolated, I do not consider any place in the United States with less than either 2000 or 4000 inhabitants in 1990 as potential neighbor for an isolated towns. To be consistent with this definition of a neighbor, an isolated town must have more than either 2000 or 4000 inhabitants. Otherwise, for a hypothetical area populated with towns with less than 2000 inhabitants, each town in this area would be an isolated town.


Second, I need to check if a town is isolated. To do this I have coded a routine in Arcview© which counts the number of towns located within a specific distance from the central place. On average, a place in the United States has X towns within a 20 mile distance. Thus, if for instance there are no towns located within a 20 mile distance from Tuba City, Arizona, then I can conclude that Tuba City is an isolated town. A town is isolated if there are no other towns located within 20, 30 or 40 miles away from it. Table I presents the number of isolated towns in the Continental United States:


Table 1: Number of Isolated Towns

No neighboring cities of a least 2000 inhabitants within Number of Cities
20 miles 449
30 miles 194
40 miles 102

Figure 1: Dayton, Washington Isolated Town


Several towns are adjacent to each other. An analogy to this situation (while of course not itself an isolated town!) is the Minneapolis-Saint Paul MSA, which is composed of two adjacent cities: Minneapolis and Saint-Paul. If I do not consider Minneapolis and Saint-Paul as a single city then I automatically count this agglomeration as having at least one neighboring town. To eliminate the problem of a single town which is split up into two municipalities, a town located within 1 mile of the central place is not counted as a neighbor.

Isolated Cities and Adjacent Towns
No cities within 1 mile
At least 1 city within 1 mile

Figure 2: Single Agglomeration composed of many places

Zip Codes


To make this data set more useful to researchers, I also select zip codes within a certain distance of the isolated towns. Zip codes can be used, for instance, to count the number of establishments within 5 miles of the central place, since ready-mix concrete plants frequently locate outside the boundaries of the municipality, and thus will not be part of the municipality proper, but will belong to a zip code located within a small distance from the central town. Again, the data on zip codes come from the U.S. Census Bureau. I include all zip codes within 5, 10 and 20 miles of an isolated town.

Figure 3: Zip Codes within 5 miles of Burney, CA

Code Book


variable name storage type variable label
areaname str25 Place Name
class str18 Place Type
st str2 State
stfips str2 State FIPS
placefip str5 Place FIPS
houseunits long Number of Housing units in
Place in 1990
population long Population of Place in 2000
pop_cl byte Place Population Class
arealand float Land Area of Place
areawater float Water Area of Place
nbr1 byte Cities with more than 2000
inhabitants within 1 mile (including place itself)
nbr20 byte Cities with more than 2000
inhabitants within 20 miles
nbr30 byte Cities with more than 2000
inhabitants within 30 miles
nbr40 byte Cities with more than 2000
inhabitants within 40 miles
autoroute byte Highway within 5 miles of place
population1 long Population in 1990
vacant long Number of Vacant Houses
housing long Number of Houses
zipcode5mx long Zip Code within 5 miles of
place number x
zipcode10mx long Zip Code within 10 miles of
place number x
zipcode20mx long Zip Code within 20 miles of
place number x

Arcview Code for finding the number of neighbors for an isolated town

‘ This script selects all cities which are located far away from other cities



‘ Get the FTabs…

cities= av.GetProject.FindDoc(“View1”).FindTheme(“placeply.shp”).GetFtab


‘Clone the cities Note that over2000 was created to select only cities with more than 2000 people






‘ 20 miles


distance = Units.Convert( 20, #UNITS_LINEAR_MILES, #UNITS_LINEAR_DEGREES )





‘ Select population cutoff say 2 000





‘start loop


for each rec in cities.GetSelection.clone


‘ Select cites within 20 miles of selected cities





cities3.SelectByFTab( cities2, #FTAB_RELTYPE_ISWITHINDISTANCEOF,distance, #VTAB_SELTYPE_NEW  )




‘ Count cities which are within a close distance to the major city which will include the city itself








‘Record number of cities within area





[1] Source: US Census Gazetteer File for Places,, accessed March 20, 2006.