USP 192: GIS for Urban and Community Planning (Spring 2007)

Contact Information

The United States Census

Census background:

The United States Constitution mandates that the census be taken at least once every ten years, and that the number of members of the House of Representatives from each state be determined accordingly (called reapportionment). In addition, Census Bureau statistics are used for apportioning Federal funding for many social and economic programs. The first census was done in 1790. Census-takers went door-to-door and recorded the number of people in each household, and the name of the head of the household. Slaves were counted, but for apportionment purposes each counted as only three-fifths of a citizen. American Indians being neither taxed nor considered during apportionment, were not counted. The first census counted 3.9 million people, less than half the population of New York City in 2000. The 2000 census counted over 281 million people.

A note on variables and geography

Variables and geography change over time! Also, there are differences between the print and raw data. For example, the 1970 printed population and housing has 24 variables. There are 259 variables in the raw data!

Short form (age, sex race data) vs. long form (sample data)

Census data overview

Sample data vs. 100% data

Summary File 1 presents counts and basic cross tabulations of information collected from all people and housing units. It includes counts for many detailed race and Hispanic or Latino categories. This file is often referred to as the "age, race and sex" data. Summary File 1 presents data for the United States , the 50 states, and the District of Columbia in a hierarchical sequence down to the block level for many tabulations, but only to the census tract level for others. Summaries are included for other geographic areas such as ZIP Code Tabulation Areas (ZCTAsâ„¢) and Congressional districts.

Summary File 3 presents detailed social, economic, and housing data to the block group level (such as place of birth, education, employment status, income, value of housing unit, year structure built) as a sample and weighted to represent the total population. Population and housing information collected from the census long form for a one in six sample of households in the United States and Puerto Rico, and on a continuous basis for selected areas in the American Community Survey. This is called "sample data".


Generally the larger the geographic area, the more variables will be recorded.

US (largest geographical area) à block (smallest geographic area)

Census tracts are the most heavily used geographical units. C ensus tracts have between 2,500 and 8,000 residents and boundaries that follow visible features. They may be divided into block groups and blocks.

Other geographical units:

Metropolitan Statistical Areas (MSA): one or more counties that contain a city of 50,000 or more inhabitants, or contain a Census Bureau-defined urbanized area (UA) and have a total population of at least 100,000

States, counties, American Indian areas, Consolidated metropolitan statistical areas (CMSAs), and primary metropolitan statistical areas (PMSAs).


Census data technical documentation

SSDC Census2000 quick links

Census holdings:

  • UCSD: all censuses in paper starting with the first in 1790.
  • SSDC: San Diego, California Historical Census Data : 1950, 1960, 1970
  • Geolytics: 1970 - 2000 Geolytics census research package on CD-ROM (ask at the SSHL reference desk for assistance). This thorough resource includes Neighborhood change database, 1990 long form in 2000 boundaries, CensusCD 1990 BLOCKS, Census reference package (1970, 1980, and 1990 long form products, Complete census 2000 package (long form, short form, redistricting, blocks).
  • Web:

Finding census data with American Factfinder:


  • Choose summary file, select "custom table"
  • Choose geographical area
  • Choose variables
  • Print/download data set

To get help with American Factfinder, click on "help" and then "tutorials" from the main page.