Widespread conversion of rural lands to urban
uses is an issue challenging all levels of government.
To provide policymakers with information useful
for projecting future changes in land use, ERS
has created a system to classify remaining farmland
into “population interaction zones for agriculture”
(PIZA). These zones represent areas of agricultural
land use in which urban-related activities affect
the economic and social environment of agriculture.
In these zones, population interactions with farm
production activities increase farmland value,
change farm enterprises, and elevate the probability
of conversion to urban-related uses.
Though closely related to the existing ERS county-level
Urban Influence Codes and census tract-level Rural-Urban
Commuting Area Codes, PIZA is a complementary
system that provides codes for much smaller 5-kilometer
squares. In addition, the PIZA codes provide a
continuous and cardinal (rather than ordinal)
measure of population interaction, which is especially
useful for some analyses.
Designation of the zones begins with use of
common Geographic Information System (GIS) software
to assign an index number to each 5-kilometer
cell in a grid laid out across the contiguous
48 States. The “population interaction index”
(PII) measures the influence that nearby population
exerts on agricultural land in each grid cell.
Each PII is a continuous measure that accounts
for both population size in all grid cells within
a 50-mile radius and their distance from the target
grid cell. The index increases as population increases,
and/or as distance between agricultural land and
that population decreases.
In order to assign cells to either a “rural”
zone or a “population interaction”
zone, thresholds for PII were established for
each of 20 Land Resource Regions (LRRs) defined
by USDA's Natural Resources Conservation Service.
Thresholds were established near the upper end
of the range of index numbers for grid cells in
the most rural census tracts of each LRR. Within
each LRR, index numbers below that threshold represent
rural levels of population interaction, which
exist even in the absence of urban-related population
interaction. Any grid cell whose index exceeds
the threshold is classified into a “population
interaction zone.” Cells initially classified
into the population interaction zone are further
classified into one of three categories, yielding
a four-level classification: rural (little or
no urban-related population interaction) and low,
medium, and high population interaction.
The indices (PII) and zone codes (PIZA), which
can be used to classify any geographic point in
the 48 contiguous States, are available on the
ERS website. GIS software is necessary, however,
to retrieve the indexes and zone codes and relate
them to any given geographic point.