Massachusetts Institute of Technology - Department of Urban Studies and Planning

11.520: A Workshop on Geographic Information Systems
11.188: Urban Planning and Social Science Laboratory

Making Sense of the Census

September 29, 2010


Census Discussion Overview - Utilizing Large Tabular Datasets (e.g., the US Census)

Example - Thematic map of Income (viz., median earnings)

What Is the Census and Why Do We Care?

How the Data Are Collected

What's Included: Information on Population, Employment and Housing Characteristics

  • Why We Need to Know the Two Components
  • Census Geography and Summary Levels

    The Census organizes and aggregates data into a series of geographic hierarchies

    Standard Hierarchy of Census Geographic Entities (from Census 2000 Summary File 1 Technical Documentation, prepared by the U.S. Census Bureau, 2001, p. A-25)


    Geographic Unit
    010 United States
    020      Region: Northeast (NE), Midwest (MW), South (S) and West (W) Regions
    030           Division:
                   Northeast Region: New England, Mid Atlantic
                   Midwest Region: East North Central, West North Central
                   South Region: South Atlantic, East South Central, West South Central
                   West Region: Mountain, Pacific
    040                      State (includes Washington, D.C. & Puerto Rico)
    050                           County
    060                                County Subdivision 
    070                                     Place 
    080                                          Census Tract / Block Numbering Area
                                             (average 4,000 persons)
    090                                               Block Group  (average 1,000 persons)
    100                                                    Block (average 85 persons)
    Summary Level Geographic Unit
    040 State (includes Washington, D.C. & Puerto Rico)
    050      County
    140           Census Tract
    150                Block Group

    Census Summary Files

    The most useful files distributed by the Census Bureau are the Summary Tape Files (now renamed simply Summary Files) that aggregate the individual census forms to various levels of census geography. American FactFinder provides a forms-based online interface to many US Census datasets including SF1 and SF3. The FactFinder website is convenient when you want data for a single census tract or a small number of areas. It is also convenient when you want a few percentages (such as percent owner-occupied) that would otherwise require downloading the numerator and denominator needed for your own calculation. If you need to download many variables or data for many areas, you may be better off accessing the core SF1 and SF3 datasets described above via the following links. (The GIS lab in the Rotch Library has many Census CDs and other third-party tools that may also be helpful. Later in the semester, we will also use the MIT Library's online geodata repository that contains direct ArcGIS access to many useful datasets including some US Census data.)

    The Census Bureau distributed the 1990 Census files as DBF files on CD-ROMs. The Census Bureau has posted the contents of many 1990 CD-ROMs online. These are available via HTTP and FTP. Also,

    In fact, the 1980 STF 1 and STF 3 are now online! You can obtain the 1980 STF 1 via HTTP or FTP and the 1980 STF 3 via HTTP or FTP. Documentation is available from the Odum Institute for Research in Social Science.

    The Census Bureau is distributing the 2000 Census files on CD-ROMs, DVD-ROMs in a proprietary format and online in flat ASCII format via HTTP and FTP.

    The 1980 STF 1 and STF 3 files had varieties similar to those of the 1990 Census.

    A Quick Look at the Census Data and Documentation

    1980 Census


    2000 Census

    Censuses in Other Countries

    More Information About the 2000 Census

    Example: Let's find the unemployment rates for Cambridge area block groups

    The four steps above that are marked with ' #' are not needed for the class exercises since we have already built an MS-Access database with the Census variables needed for the lab and homework exercises.

    This data extraction and mapping exercise is complicated because the datasets are so large and include so many variables and geographic identifiers. But it is illustrative of the issues and steps involved in (a) understanding very large and highly structured datasets, and (b) using desktop tools to find, download, and mix-n-match geometry and tabular data from different online sources.

    Note that the US Census provides many online tools to obtain census data. Likewise, there are many third-party tools and CDs that repackage the data in smaller chunks, with or without maps, and sometimes in pre-processed forms (e.g., after normalizing to percent owner-occupied rather than just as the raw counts). These assorted tools fill many nitche markets. Relatively few census data users understand the data structure and raw files at the level described in these lecture notes - i.e., at the level needed to find and use any of the thousands of columns of data that are available at each level of geography..

    The section of these notes entitled "Introduction to the U.S. Census of Population and Housing" is adapted from a Microsoft PowerPoint presentation originally
    created by Prof. Qing Shen for 11.208 on January 21, 1997.
    Augmented and modified 1999-2010 by Thomas H. Grayson,
    Anne Kinsella Thompson, Sarah Williams, Xiongjiu Liao, Joe Ferreira, and Shan Jiang.

    Last modified: 29 September 2010 [shanjang]

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