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Building Blocks
The process of creating decennial census tabulation blocks.
GeoElections User’s ConferenceOctober 6th, 2011Tampa, Florida
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Background
• Census Block– The smallest entity for which the Census
Bureau collects and tabulates decennial census information; bounded on all sides by visible and nonvisible features shown on Census Bureau maps.
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Background
• Enumeration Districts (EDs)–1870 to 1980– 1870 - Maximum size of sub-districts for
enumeration lowered from 50,000 to 6,000– 1880 – Maximum size of sub-districts for
enumeration lowered from 6,000 to 4,000– 1910 – began using legally or administratively
defined entities for EDs– 1940 – Blocks introduced for larger places
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Background
• Enumeration Districts (EDs)–1870 to 1980– 1940 – Blocks introduced for larger places
• Housing data• Blocks in cities with populations of 50,000+ as
measured by the previous census• 191 cities in 1940
– 1950 • 209 places in 1950
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Background
• Enumeration Districts (EDs)–1870 to 1980– 1960
• Total block population reported• 295 cities in 1950• Additional 172 places paid for block level data
– 1970 – first large scale enumeration using mail rather than door to door enumeration
• Block coverage expanded to cover the expected extent of all 1970 urbanized areas
• Contract block areas grew to 966
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Background
• Enumeration Districts (EDs)–1870 to 1980– 1980
• Blocks expanded to all incorporated places with a population of 10,000 or more
• 5 states contracted for full state coverage of blocks (GA, MI, NY, RI, and VA)
• Blocks covered 78% of the population and 7% of the land area
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Background
• Current configuration of statistical geography -1990 to present– 1990
• TIGER (Topologically Integrated Geographic Encoding and Referencing System) database is developed
• Nationwide block coverage established• Local census statistical areas committees
participate in delineating block groups and tracts
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Background
• Current configuration of statistical geography -1990 to present– 1990 (continued)
• Redistricting Data Program introduces the block boundary suggestion project
• Automated block creation and numbering introduced
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Background
• Current configuration of statistical geography -1990 to present– 2010
• MAF/TIGER Accuracy improvement project performed prior to 2010 Census
– Incorporated new features– Spatially realigned TIGER database to a higher level of
precision
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Creation of tabulation blocks
• Automated process – the “Tabulation Block Algorithm”– A series of criteria for formulating blocks
(some of the considered criteria)• Geographic area boundaries• Feature type• Addressability of features• Block Boundary Suggestions• Previous census block boundary
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Creation of tabulation blocks
• Geographic tabulation area boundaries are always held as block boundaries.
• State & County boundaries• Incorporated Place• Minor civil division boundaries• Census designated place (CDPs) boundaries• Statistical areas (Census Tracts and Block Groups)• Certain Landmark boundaries (Military bases)• Et al.
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Creation of tabulation blocks
• Certain features are also always held as block boundaries.
• Roads classified as Primary Roads by MAF/TIGER Feature Classification Code (MTFCC)
• Roads classified as Secondary Roads by MTFCC• Redistricting Data Program Block Boundary
Suggestion Project “Must Holds”• Tribal Census Block Definition Program “Must
Holds” (On tribal lands only)
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Creation of tabulation blocks
• Other features can be used as block boundaries but are not always (examples)– Local Roads
– Vehicular trails
– Alleys
– Railroads
– Streams (Perennial Only)
– Powerlines
– National Park Boundaries
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Creation of tabulation blocks
• Features can be classified as “Do Not Hold”– Unique to the Redistricting Data Program
– Feature can not be classified as “Do Not Hold” if needed for a geographic tabulation area
• All features and boundary types deemed as eligible to be a possible block boundary are given a priority value.
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Creation of tabulation blocks
• All features deemed eligible are used to make a lattice of possible blocks
• If a polygon’s boundaries are all required as block boundaries, it becomes a block
• Remaining blocks are then evaluated for merging.
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Creation of tabulation blocks
• Remaining block evaluations for merging– Minimum size
• Iterative check accounting for size and bounding feature priority values
– Single edge block– Addressable Feature
• Bounding or internal
– Isolated water block– Adjacent block qualifications
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Creation of tabulation blocks
• Applying the block numbers– If the number of final blocks in a block group
exceeds ~1000 then more merging occurs• Water blocks first• Lower priority edges
– Block numbers assigned in a serpentine fashion starting in the northeast corner
• First digit is block group number• Start at 000 and go until 999 or all block numbers
have been assigned
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Creation of tabulation blocks
• Where issues can arise– Feature attribution and classification– Connectivity of features in the feature network– Positional Accuracy
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Influencing Future Blocks
• Participant Programs– Participant Statistical Areas Program (PSAP)– Boundary and Annexation Survey (BAS)– Redistricting Data Program (RDP)– School District Review Program (SDRP)
• Initiatives– Geographic Support System Initiative (GSS)
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Influencing Future Blocks
• Participant Statistical Areas Program – Tract and Block Group boundary definition– Define or modify Census Designated Places– Local Census Statistical Areas Committees or
Census Regional Offices act as the participant– Pre-decennial program
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Influencing Future Blocks
• Boundary and Annexation Survey (BAS)– Legal boundaries of incorporated places– Ongoing annual program– Partial and Full BAS years– Highest Elected Official is the invited
participant
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Influencing Future Blocks
• Redistricting Data Program– State Legislative and Congressional district
boundaries – split blocks allowed– Voting Districts – new boundaries allowed– Block Boundary Suggestion Project– Non-partisan state liaison appointed by
governor and legislature
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Influencing Future Blocks
• School District Review Program– Elementary, Secondary, and Unified School
District boundaries– Not as applicable to Florida due to county
based school districts– State liaison
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Influencing Future Blocks
• Geographic Support System Initiative– Focused on updating and maintaining the
Master Address File– Includes components for updating and
maintaining geographic features– Still in the research and design phase
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Key Points
• Consistent participation in ongoing programs such as BAS
• Coordination within states on state based programs
• Participation in opportunities that arise from the GSS Initiative
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James Whitehorne
Assistant Chief
Redistricting Data Office
U.S. Census Bureau
www.census.gov/rdo
301-763-4039
26