Overview
Widespread conversion of rural lands to urban uses has drawn
attention at all levels of government. To provide information
useful for projections of 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 (residential, commercial, and industrial) affect the
economic and social environment of agriculture. In these zones,
interactions between urban-related population and farm production
activities tend to increase the value of farmland, change the
production practices and enterprises of farm operators, and elevate
the probability that farmland will be converted to urban-related
uses.
The PIZA codes are derived from a classification scheme that
indexes small geographic areas according to the size and proximity
of population concentrations. Designation of the zones begins with
use of common GIS software to assign an index number to each of
many small (five-kilometer) grid cells laid out (figuratively)
across the contiguous 48 States. These "population-interaction
indexes" (PII) are designed to provide a cardinal measure of the
potential interaction between nearby urban-related population and
agricultural production activities in each grid cell. (By a
cardinal measure, we mean that the codes effectively rank each
location or area on a continuous scale.) The population-interaction
indexes are based on the regional economist's or geographer's
concept of a "gravity" model, which provides measures of
accessibility to population concentrations. This model measures
population interaction by accounting for the size of all
populations in the proximity of a given location or grid cell and
the distance of that location or grid cell from those
populations.