Those of us who were part of the Alpha Test of the TAZ-UP (TAZ definition
software) are now reviewing the first draft of the User Guide. One item that
I am trying to draft right now is about why a TAZ for CTPP may be different
than a TAZ that you have for your regional model.
This is my current draft, which I am sure will change, with your input. Also,
IMPORTANT NOTE--we still have not heard from the Census Bureau Disclosure
Review Board what kind of population threshholds or "average" populations they
may require for tabulation.
Oh, and finally, the plan is for the TAZ submissions to Census Bureau to occur
around June/July of 1999. Then the TAZs will be added to TIGER/Line 99, which
will be distributed for you around early 2000 to validate that what you
submitted was correctly input to TIGER.
Standard Census products use standard Census geography. That is, tabulations
are prepared by State, county, munipality, census tract, and block group.
The CTPP allows users to define their own geography. In this case, traffic
analysis zones (TAZ). A TAZ defined for CTPP may differ from a TAZ defined
for a local model.
Reasons why this may occur:
1. A TAZ defined for CTPP 2000 is required to be composed of the smallest
allowable polygons in the Census TIGER/Line file. Typically, in the
TIGER/Line file, block boundaries are limited to physical features such as
streets, power lines, streams and rivers, etc. TIGER/Line does NOT have parcel
boundaries. A local GIS may have more detailed geography, e.g. parcels and
lot lines that allow TAZs to be defined by unique parcels. You will want to
try to make your TAZs for CTPP 2000 as close to your own TAZs as possible.
Issues: You may want to include businesses on both sides of a street. You
cannot create another TIGER line segment to break up the smallest allowable
polygon to capture both sides of the street. You are limited to selecting the
entire smallest allowable polygons on both sides of the street segment.
DRAW A PICTURE HERE.
2. Small TAZs with little or no resident population or employment
population. For your local model, you may have defined TAZs where you expect
future growth to occur. This makes it better for you to display your
forecasted data to 2010 or 2020. However, for the tabulation of Census
responses in year 2000, for areas with very small populations, there is a risk
that the data will be suppressed by the Census Bureau. You may want to
aggregate areas to create somewhat larger TAZs for CTPP so that you can take
advantage of the large sample (1 in 6 households nationwide) from the
decennial long form. That is, you will get data reported in the CTPP for a
larger geographic area, and will have to disaggregate it to your *real* TAZs
Similarly, if ALL your TAZs are defined with very small populations, you risk
the chance that likewise, ALL your data will be suppressed by the Census
Bureau. For example, your local model has 300 zones, but the average
population or employment is less than 300 persons. You run a high risk of
having all your data suppressed due to confidentiality requirements of the
Census. For the CTPP, you may want to have a *super-district* system that
reduces the number of zones to 150 zones. With your best guess, this
increases the average population and employment to 500 persons. This would
likely not be suppressed by the Census Bureau.
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