US Census Thomas Talbot February 5, 2013
Outline Census Geography TIGER Files Decennial Census - Complete count American Community Survey Yearly Sample Obtaining Data - American Fact Finder - Census FTP Site - Interactive Maps Data Quality
The US Census is mandated by the Constitution. The population is enumerated every 10 years. Results used to allocate Congressional seats, and government funding. The Census Bureau is responsible for the Census. The first census was taken in 1790, under Secretary of State Thomas Jefferson. For years between decennial census, the Census Bureau makes estimates using the American Community Survey.
US Census TIGER Files Topologically Integrated Geographic Encoding & Referencing Digital database developed to support Census s mapping needs. The data base defines the location and relationship of streets, rivers, railroads, and other features to each other.
TIGER Features Points: Land marks Lines: Roads Streams Railroads, etc. Polygons: Census tracts Towns Counties School districts Congressional districts etc.
Census geographic hierarchy hierarchical tabulation systems, e.g.: USA Region Division State County Tract Block Group Block 2000 Census tallies for entire US: 65,443 tracts 208,790 block groups 8,205,582 blocks U.S. Census Data & TIGER/Line Files
Census Units # New York State 2010 Blocks 350,169 Block Groups 15,241 Tracts 4,902 Towns (MCDs) 1,013 Counties 62 Places 1,189 ZCTAs 1,794
Census Geographic Relationships
Census Geography, Albany County
Census Geography, City of Albany
Census Tracts Census tracts are small, statistical subdivisions of a county. Uniquely numbered in each county with a numeric code Census tracts average about 4,000 inhabitants Minimum Population 1,200 Maximum Population 8,000 11
Updating Census Tracts Census tracts are designed to be relatively permanent over time. Any changes are documented so data can be compared from decade to decade. Census tracts with more than 8,000 people are split into 2 or more tracts Census tracts with less than 1,200 people are merged with a neighboring tract Small boundary corrections are sometimes allowed as well. 12
Census Tracts are Merged as Population Decline below 1,200
Decennial Census Basic demographic information Age Sex Race Ethnicity Number of Housing Units Group Quarters Household relationships
American Community Survey A large, continuous demographic survey 250,000 Households nationwide every month Housing unit address is the sample unit not people Produces annual and multi-year estimates of the characteristics of the population and housing
American Community Survey Sample of households. Census views it as producing characteristics distributions, NOT counts Produces information for small areas including tracts, block groups & population subgroups
American Community Survey
Multiple numbers for the same area Data Release Every Year Period rather than point estimates 2010 release for areas of 65,000 or more 1-year estimate (2009 collection) 3-year estimate (2007-2009 collection) 5-year estimate (2005-2009 collection) 2010 release for areas of 20,000 to 65,000 3-year estimate (2007-2009 collection) 5-year estimate (2005-2009 collection) 2010 for areas areas under 20,000 5-year estimate ONLY (2005-2009)
FTP S
FTP Site Provides Comma Delimited Data which can imported into spreadsheet, statistical or GIS software
Census Data Quality Accuracy of census counts Spatial Accuracy Temporal Accuracy
ACS Statistical Quality Impute missing data ACS products (sample) Reported at 90-percent confidence level Summary Files also have Standard Errors May affect uses of the data Denominators Regression analysis
The census does not always put populations in the correct census area. Merge these blocks before using aggregation tool From 2000 census 38
Census Enumeration Errors 831 persons living in Colonie Shopping Center 0 persons living in neighboring apartments
Intercensal Estimates are Unreliable example: several disease cases have recently occurred in this area, the intercensal Claritas estimate = 0 population New construction Census 2000 0 Persons 2008 Intercensal Estimate 0 persons 40
Temporal Accuracy TIGER Streets Real Property Points
Spatial Accuracy Census TIGER Files (2000)
New York ALIS Street Files (2005)
Homework Read: Chapter 3: Cromley & McLafferty, GIS & Public Health Mills JW, Curtis A, et al. The clearinghouse concept: a model for geospatial data centralization and dissemination in a disaster. Disasters, 2008 32(3): 467-479 Explore: US Census Web site: www.census.gov New York State GIS Clearinghouse: http//gis.ny.gov The End