The Census Bureau Geography Division has asked us to submit to them
software requirements for TAZ definition by the end of Calendar Year
2006. We are working on the assumption that the CTPP pooled fund
proposal will move forward through AASHTO's process.
In 1998, FHWA contracted with Election Data Services to produce the
TAZ-UP for software creating the 2000 TAZs used for CTPP 2000. TAZ-UP
greatly simplified the ability to create and export TAZs to CB's TIGER
files. The process was a big improvement over the paper-map-pencil
drawings, which were used for the 1990 CTPP.
For the 2010 Census, the CB has a contract with M-cubed and its
subcontractor Caliper Corporation for software development to support
the "Participant Statistical Areas Program" (PSAP). The PSAP includes
the tract and block group definition process. The software being
developed for this program can be modified to accommodate TAZ, SuperTAZ
or any other geographic units that the transportation planning community
would like to have added into TIGER.
In our last regular CTPP meeting, we did discuss concepts of corridors
and downtowns, but we think those would be best left as aggregates of
TAZs that are handled later through a TAZ-equivalency process for CTPP,
rather than adding them into TIGER.
Below is a summary of our discussions from a meeting held on October 18,
2006. PLEASE LET US KNOW IF YOU THINK WE ARE MISSING SOMETHING
IMPORTANT! Please send your remarks to nanda.srinivasan(a)dot.gov by
FHWA Office of Planning
Summary of Specifications
The software should have the ability to create 3 levels of TAZs - one is
a base TAZ which will be subsequently used to define two other larger
size aggregations (optional by county). The base TAZ would be similar
to the traditional "small area geography" TAZs in 2000. The software
must provide ability to aggregate these TAZs to medium size (about 4,000
population) and larger size (about 20,000 population) aggregations (much
like Block Groups being aggregated to Tracts, or Tracts aggregated to
PUMAs), if desired by the MPO/State.
The functional specifications include:
1. Load TIGER/Line 2008 or 2009 files in their native format,
including TAZ, Tract, Block Group, Roads, water, and other polygon
2. Display TIGER layers.
3. Import and display background layers:
a. Import and display of other MPO or State owned data
(shapefiles - points, lines, or polygons), as long as these files are in
decimal degrees, NAD 83 projections.
b. Import and display of aerial photography, as long as
these files are in decimal degrees, NAD 83 projections.
4. Create or Devise Traffic Analysis Zone (TAZ) polygon features
using screen digitizing starting from:
a. 2000 TAZ geography, if defined, or else current Tracts
or Block Groups.
b. Using primary polygons in TIGER, if 2000 TAZs were not
defined, or if the agency does not choose to use Tracts or Block Groups
as starting point.
c. The software should have the ability to aggregate
polygons using a lassoing function (to zoom in or out), select polygons,
or draw a line around (point-by-point) and add them to a TAZ.
5. TAZ Definition Guidelines:
a. The software should prompt the user to create TAZs to
cover the whole county without any discontiguity. Importantly, there
should not be any sliver TAZs or duplicate TAZ numbering in any county.
b. The maximum number of alphanumeric characters for TAZ
might be set at 8.
c. There may be cases where a TAZ boundary might require
segment(s) to be added to TIGER. If there is a valid geography that
does not show as a line segment (e.g.: a polygon bounded by a new road),
the software must provide the ability to use the line segment as a TAZ
boundary, and allow the user to print the view/map, annotate it, and
submit it to electronically/via hardcopy to the Census Bureau.
Count and display
* 2000 total residential population (using 2000 block population
* 2000 resident worker population (workers by place of
residence), and 2000 workers by place of work (using 2000TAZ/Block Group
values) for each delineation of a TAZ. NOTE: CTPP Part 2 should be
used for place of work counts. TAZ and Block Group counts are not
available nationwide, although tract counts are available nationwide.
* If the currently defined TAZ cuts across 2000 geography
(TAZ/BG), then use proportion of TAZ by area in that BG/TAZ to estimate
resident/worker counts. Because this task might be difficult, a
separate cost estimate should be requested from the software developer.
Count and display the sum of resident workers, and workers by place of
work to aid to determine the size of the TAZ.
6. Final Checks: Once TAZ definition is complete, and before
export, the software must check for items 6a through 6e, and flag the
items on a list. Export should be allowed only if all the following
items are flagged. If any of these items are not satisfied, the
software should loop the user through each entry.
c. Uniqueness: Ensure no duplicate TAZ numbers anywhere in
d. Check for polygon closure for each TAZ.
e. Distinguish whether the user wants the TAZ boundary to
move with a tract/BG boundary (since the tract boundaries will be draft
at that time), or if the user wants the TAZ boundary to be locked and
not move with a tract/BG boundary.
7. Export output: Upon completion of the TAZ definition and after
completion of final checks (Step 6), the software should provide ability
a. Export the file to the requirements of the Census
Bureau, Geography Division.
b. Export the final TAZ shapefile copy for MPO/State use.
8. (Optional by County) Aggregations of TAZs. Once basic TAZs are
developed, the software must allow the ability to aggregate these TAZs
into two other transportation geographies - A medium sized TAZ (Med
TAZ), and a large TAZ (Lg TAZ).
a. Use final TAZ shapefile as starting point to develop
medium size (Med TAZ) or Large size (Lg TAZ) aggregations.
b. The software should have the ability to aggregate
polygons using a lassoing function (to zoom in or out), select TAZ
polygons, or draw a line around (point-by-point) and add them to MZ or
c. Provide counters similar to item 5c to check the
population/employment of these zones.
d. Prior to completion and export, do final checks similar
to item 6.
e. Provide export outputs similar to step 8.
B. Provide the final software on CD-ROM or through a web-based
C. Provide a "Help" button to explain software features.