The database divides
the earth into Big Cells, rectangular grids of 20 minutes of
latitude by 30 minutes of longitude, an area of around 2,000
square kilometers at the equator. Some country data are
more detailed. The smaller Mini Cells make rectangular grids
of 5 minutes of latitude by 7.5 minutes of longitude.
There are 6 Big Cells in a "square" degree, and each Big
Cell contains 16 Mini Cells. The location of
each cell is identified by the latitude and longitude of its
center point.
Population data is primarily obtained through censuses, surveys and maps
from the Bureau of the Census Library or from the countries themselves.
Detailed data are obtained by working from the national totals towards
the
first-order administrative levels; typically state, province or
region. From there, data are matchedto the county or
department level (second level) and then to districts or
arrondissements (third level). The ideal level for the
purpose of geographical distribution is the fourth level,
that is, towns and villages. Data at the fourth level are
not often available in a uniform coverage.
- Rural Population
- The rural population is distributed to a set of rectangles
called Big Cells. Medium-scale (typically
1/200,000 to 1/50,000 scale) maps are used to identify
individual towns and to estimate the proportion of each unit
within each cell. Populations of the places falling within
cells are then summed and projected.
- Urban Population
- Populations of urban agglomerations
of 25,000 people or more are are distributed to a set of
circles. City plans and large-scale maps are used to
determine areas of highest population density. The entire
urban area is then encompassed by circles whose radii
range from 0.3 to 2.0 nautical miles. Circles are
identified by the coordinates of their centers.
Population estimates are made for each circle and then
projected for 12 years.
- Total Population and Density
- The population of any
urban agglomeration summed with the total rural population
of the rectangular area comprises the area's total
population. Density is computed using the area of the
rectangle, correcting for water coverage.
- Projected Population Data
- Urban and rural populations are projected by extrapolating the rate
of
change in their ratio to total country population. Projections of the
total population are based on detailed data for the three
components of population change: births, deaths, and
migraion.
In 1993, IPC completed work for the United States country database
based on the 1990 census. This small area population database is
described in more detail in an article presented by Robert Leddy ("Small Area Populations for the
United States") at the 1994 Annual Meeting of Association of American
Geographers. Map images are accessible via links within Leddy's article,
or
by accessing this
image library.
For more detailed information on the Global Population Data Base,
including
a description on the use of these data involving Spain and Sweden,
consult
Chapter 4 in the ISSC "Population Data and Global Environmental
Change"
report. To browse the entire ISSC report,
click here.
