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 as Western 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	374
At least 1 city within 1 mile	75

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

cities2=cities.clone

cities3=cities.clone

' 20 miles

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

close = Units.Convert( 1, #UNITS_LINEAR_MILES, #UNITS_LINEAR_DEGREES )

' Select population cutoff say 2 000

expr="[population]>2000"

cities.GetSelection.ClearAll

cities.Query(expr,cities.GetSelection,#VTAB_SELTYPE_NEW)

'start loop

for each rec in cities.GetSelection.clone

' Select cites within 20 miles of selected cities

cities2.GetSelection.ClearAll

cities2.GetSelection.Set(rec)

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

allneighbours=cities3.GetNumSelRecords

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

cities3.SelectByFTab( cities2, #FTAB_RELTYPE_ISWITHINDISTANCEOF,close, #VTAB_SELTYPE_AND )

nearneighbours=cities3.GetNumSelRecords

'Record number of cities within area

cities.SetValue(cities.FindField("Nbr20"),rec,allneighbours-nearneighbours)

end

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[1] Source: US Census Gazetteer File for Places, http://www.census.gov/geo/www/gazetteer/gazette.html, accessed March 20, 2006.