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Assumptions
A
number of assumptions were made in order to be able to estimate
buildout. No buildout is perfect, just as no model is a perfect
representation of the real world. In this buildout analysis,
uncertainty is minimized by analyzing individual parcels.
This method has advantages and disadvantages; a comparison
of the parcel method with other methods used for buildout
is available.
The
major assumptions in this analysis and their potential impact
on the results are listed below. It is important to note that
inaccuracies in the source data for this analysis add a significant
uncertainty to the calculations for each parcel. This is due
to differing accuracy in the data sets, and the geocoding
of the data sets not being perfectly aligned with one another.
These errors are not systematic, therefore they will both
increase and decrease the number of possible units. For example,
if the wetlands coverage shows the correct size of a wetland,
but its position is off by 50 feet, the wetland may appear
to be in a "wrong" parcel. The aggregate impact
of this error will be to increase the number of units that
can go in the first parcel and decrease the number of units
that can go in the second. This does not impact the overall
number of units that are estimated in full buildout, but it
does impact where those units are.
The
following is a list of the major assumptions that were necessary
to simplify the analysis and the potential impact of these
assumptions. If the assumption tends to increase the number
of units, its potential impact is overestimation, if it tends
to decrease the number of units, its potential impact is underestimation.
Constrained
Land: The constrained land coverage was used consists
of wetlands, wetlands buffers, steep slopes, and flood areas.
There are a number of types of constrained land, as defined
by Charlestown's zoning regulations, that can not be included
due to lack of good data sources. These include the following
constraints -click on the link for an explanation of the data
sets that were considered to be used but ultimately excluded:
Unique Cultural Sites:
The
data sets that are available for cultural sites are the RIGIS
Historic Sites and Historic Candidate Sites coverages.
These coverages are not sufficiently comprehensive to represent
accurately how this constraint would be applied to building
in Charlestown.
Rare Species:
Even though there is a Rare
Species coverage on the RIGIS database, rare species were
still not included in the analysis. The resolution for this
coverage is 3 acres, which is too large to use for buildout.
When a parcel is built on in Charlestown, there is a more
detailed investigation of rare species that determines the
amount of land that is considered constrained. Additionally,
most of the areas that contain rare species, are also in wetlands,
which are considered constrained in Charlestown and accounted
for in that way.
Ledges: There are no data available for ledges in Charlestown.
Rocky Areas: The RIGIS
Soils coverage does have an attribute for rocky areas.
The coverage groups stoniness into three categories of the
percentage of the surface covered by stones. Charlestown had
very few areas that were not constrained for other reasons
and had 10%-30% of the land covered in stone. Additionally,
because it is difficult to tell what parts of the land were
covered, it is be hard to know the actual impact of stoniness
on a parcel.
Areas That Contain Health Risks: There are no data
available for health risks in Charlestown.
These constraints
were investigated, and none of them appear to have a significant
impact on the total number of units at full buildout. These
limitations do however have significant impact on the evaluation
of individual sites.
Area
Removed for Roads: When a parcel is subdivided in this
analysis, 15% of the buildable land is taken away for roads.
The percentage of land lost to roads in a subdivision was
estimated by the Charlestown Planner, Nancy Hess in a recent
analysis of existing subdivisions. A sensitivity analysis
was conducted to determine how much the buildout would be
impacted by a change in the percentage taken out for roads,
. Reducing the percentage of area taken out for roads in every
subdivision by a third only had a 3% impact on the total number
of units in the town at buildout.
Lot
Frontage Omitted: Lot
frontage requirements in Charlestown range from 120 feet
to 300 feet. Omitting lot frontage from the calculations tends
to overestimate the number of units that can be built in the
parcel. Judging by a visual survey of the lots that will be
built, many are large and are in two and three acre zones.
In these lots, new roads will have to be built for the development.
The necessary frontage will be taken into account when these
roads are built, minimizing the frontage as a limiting factor.
In light of this, I do not think that omitting frontage has
caused a large overestimation.
Frontage was not taken into consideration when determining
the number of units that could be placed on each lot. This
was due to a technical obstacles in determining the amount
of frontage possible. Determining the possible amount of frontage
in a lot is one of the aspects of this buildout that can be
improved by further research.
Lots
Not Allowed to Join To Form Larger Lots: I assumed that
adjacent lots would not be joined to form larger lots. For
example, if there are two contiguous 2 acre lots, in a 3 acre
zone, both lots were considered unbuildable. I do not
think that this assumption had a large impact on buildout
because, judging by a visual assessment, there were not very
many lots that were too small to be built on under current
zoning. Most residential lots could have some development
and I believe that it would be unlikely that lots would
be combined under these conditions.
Land
Currently Preserved as Openspace Remaining Unbuilt: I
assumed that land that is currently preserved as openspace
will not be developed. This includes land that was set aside
as openspace during cluster subdivision. It would take major
changes in policy to allow land already set aside to be built
upon. The impacts of this assumption on the total uncertainty
of buildout are minimal.
Only
Building Single Family Homes: I assumed that only single
family homes would be built. Over the past 10 years, 96% of
the development has been single family homes. I believe
that this trend will continue, and only single family homes
will be built.
Existing
Units: The tax assessors land
classification codes delineate parcels as either having
one unit, or having multiple units. It is not possible to
tell precisely how many units are in the parcel if there is
more than one unit. Two estimates
were made for the number of units that will units that already
exist on in a particular unit. For example, if a parcel was
coded as "02" which means that it has 2-5 families
one estimate was made assuming it has the minimum of 2, and
another estimate was made assuming it has 4 units. The maximum
number of units used was not 5 because it is highly unlikely
that all multifamily parcels have the maximum of 5 units in
Charlestown. The maximum number of existing units found was
4,594 and the minimum was 4,190. These estimates give a minimum,
or maximum number of units that may exist in the a parcel
that is already built on depending on its tax code.
Parcels
That Can be Subdivided, Will be Subdivided: Parcels that
currently have a structure on them, and have room for more
development, are assumed to be subdivided. This means that
although the structure might have to be moved to fit another
house on the property, another house will be placed on the
property. No data is available to determine where on a piece
of property a house is, and therefore it is impossible to
determine if that house would have to be moved in order to
subdivide the property.
Tribal
Lands: Tribal lands
are not considered in this analysis. There may be development
on the tribal lands, but the town has no control over this
development. Also, there is very little information available
on the land that is built in the tribal zone. The tribal zone
could have a major impact on the town if it becomes largely
developed, but the potential for development is very uncertain.
No
Additional Openspace: This projection assumes that there
will be no additional openspace in Charlestown. This is highly
unlikely, and leads to an overestimate of the results.
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