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© The Nature Conservancy
| Author: |
Arlene Olivero |
Co-Author(s): |
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| Contact Info: |
Eastern Conservation
Science
11 Ave de Lafayette 5th Floor
Boston, 02111
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Phone: |
617-625-9836 |
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Email: |
arlene_olivero@tnc.org
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| Created: |
2003-03 |
Added to collection: |
2003-07-28 |
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| Geographic
Extent: |
Regional |
GIS Applications: |
Data management |
| | This map shows the Northeast U.S. by 2020 Housing Unit
Density with census block groups highlighted if they were predicted to
change density categories between 1990-2020. Housing density categories
are taken from Theobald 2001 as Rural = 0-0.025 housing units per acre,
Exurban = 0.025 - 0.01 housing units/acre, Suburban = 0.1 - 1 housing
unit/acre, and Urban 1+ housing units/acre. Red census block groups were
predicted to change into the Urban category by 2020. Orange census block
groups were predicted to change into the Suburban category by 2020. Yellow
census blocks were predicted to change into Exurban category by 2020.
Block groups that were predicted to stay Exurban between 1990-2020 are
light green. Block groups that were predicted to stay Rural between
1990-2020 are dark green.
The map highlights the high level of
current and future development within the North Atlantic Coast and Lower
New England Ecoregions. Notice the spread of suburbanization on the fringe
of previously developed areas along the coast and along major interstate
routes. Although large areas of the Northern Appalachian and High
Allegheny Plateau Ecoregions are predicted to remain in rural condition
through 2020, the data suggests certain areas in eastern Pennsylvania,
upstate New York, northern Vermont, northern New Hampshire, and coastal
Maine will see increased development pressure.
The housing density
predictions are the result of the Smith Fellow Project by David M.
Theobald "Mapping historical, current, and future housing densities in the
US using Census block-groups." 2001. This nation-wide dataset provides
housing unit density for the U.S. Census block groups in 1960-1990 and
estimates for 2000-2040. Theobald derived this data from Question H17 from
the STF3 Census Bureau long form questionnaire which codes the responses
to the question: approximately what year was your house built? These
responses were aggregated to each decennial year so that the number of
housing units between 1940 and 1990 could be mapped. To correct for
potential 1940-1990 block group aggregation errors, Theobald corrected
historical housing unit estimates for block-groups using county-level
estimates from historical decennial census per Hammer et al. 2001. To
preclude overestimation of units, he used an iterative technique that
calculates the number of underestimated housing units in each county, then
spreads them equally over the block-groups in a county, weighted by the
number of units in each block-group. The spreading function was
constrained so that the number of units in a decade did not exceed the
number of units in the next decade.
To forecast future growth
patterns, Theobald used a simplified version of the
supply/demand/allocation (SDA) model. A critical factor in accurately
portraying the spatial pattern of growth, however, is to consider the
maximum density that an area will attain. The question answered here is:
at what density does a given area (block-group) fill up with housing
units? This determines at what point new development "spills-over" into
adjacent areas. In lieu of detailed information such as zoning on
build-out densities, one alternative is to assume that future development
will continue to occur in a similar pattern as it has in recent years. The
approach used here is to assume that in any given decade, a block-group's
density will not exceed the average density of its neighboring
block-groups.
A common method to generate demand for development
is to use population projections. Theobald developed his own projections
using state-level population projections to the year 2025 from the US
Census Bureau. He then projected growth for each county out to 2025 using
their 1990-99 growth rates, but constrained them so that the sum of the
county population did not exceed the state-level projection from the
Census data. Population estimates for 2050 were derived by a simple linear
extension of growth from 2025 to 2050 to equal the same additional people
as had occurred from 2000 to 2025.
For more information, please
see "Technical description of mapping historical, current, and future
housing densities in the US using Census block-groups." David M. Theobald
Natural Resource Ecology Laboratory Colorado State University. 2001.
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