building blocks the process of creating decennial census tabulation blocks. geoelections user’s...

Building Blocks

The process of creating decennial census tabulation blocks.

GeoElections User’s ConferenceOctober 6th, 2011Tampa, Florida

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.

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

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

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

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

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

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

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

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


• 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.


• 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)


• 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


• 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.


• 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.


• 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


• 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


• Where issues can arise– Feature attribution and classification– Connectivity of features in the feature network– Positional Accuracy

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)


• 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


• 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


• 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


• School District Review Program– Elementary, Secondary, and Unified School

District boundaries– Not as applicable to Florida due to county

based school districts– State liaison


• 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

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

James Whitehorne

Assistant Chief

Redistricting Data Office

U.S. Census Bureau

www.census.gov/rdo

[email protected]

301-763-4039

26

http://www.census.gov/rdo

mailto:[email protected]

building blocks the process of creating decennial census tabulation blocks. geoelections user’s...

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