About Census Data

The US Census of population and housing is an important resource for understanding places. The census provides information on diverse topics from population density, to rents and mortgages, to commuting modes and times -- thousands of variables that can be mapped. This document provides some background on what census data are and provides some tips for GSD students and faculty on how to get census data.

Census Geography

In order to protect the confidentiality of individuals, the bureau releases only summary statistics for geographic areas: blocks, block-groups and tracts.

Population Density at three levels of geography

Census Blocks In urban areas, census blocks conform approximately to what we think of as city blocks. At this fine level of geography, the census only releases a subset of the data short-form questionaire.

Block Groups These areas are supposed to contain approximately 1200 people, but the actual count of people per block group varies widely. All of the short and long-form data is summarized at the block-group and tract level.

Tracts Tracts are larger than block groups.

The Decennial Census Survey

The US Bureau of the Census is responsible for collecting information used to redetermine allotments of governmental resources (including congressional representatives and education funding.) The constitution of the U.S. requires that in years ending in 0 each household of the united states receives a questionnaire (short form) regarding income, ethnicity and housing. The houdhold survey information from the deccennial census are aggregated to block level and larger geographies.

References

American Community Survey

In an effort to increase the frequency of "snapshots" of the american demographic landscape, the bureau began, in 2005 to collect data on a yearly basis. Rather than sending a survey to every household this program, called the American Community Survey, sends a very detailed survey to approximately about 1.7 percent of households each year. Naturally this sampling procedure produces wole-population estimates that are subject to error, and the confidence decreases for finer-grained geographies. Therefore data from a one-year sample is released only for Places having populations of 100,000 or greater. According to their methodology, by strategically comparing surveys from one year to the next, the census is able to increase the precision of the estimates, such that after 5 years of collection, estimates are released at the block-group level of agregation, along with estimates of precision for each measure.

References

Exploring and Downloading Census Data

The bureau of Census has several tools for viewing and downloading data. Unfortunately they all seem tohave been designed by someone who has no knack for interface design. Thankfully, the data are no encumbered by copyrights, and so the private sector has stepped up to the plate to produce decent web tools for exploring the data through on-line maps and downloading data.

Social Explorer

Folks who have a Harvard ID and PIN can go to Social Explorer (courtesy of Harvard University Libraries.) The process for downloading data form social explorer is similar to that of American Factfinder. One drawback is that Social Explorer does not include the block level Summary File 1 data.

Harvard users may use Social Explorer to download tables from the census. These reports come in a text file formatted as Comma Separated values. Before this can be opened in ArcGIS, the table should first be opened in Excel. A quick data dictionary can be created by taking the first two rows of column headers and pasting them into a new worksheet using the Paste-Special > Transpose option. Before saving the main table for import into arcmap, the first row of long column names should be deleted in and the GEO_FIPS column should be converted to from numeric values to text.

In ArcMap, you must create a new text column and use the field calculator to calculate the geo_fips values of your new column. Then it will be ready to join to the blockgroup geometry.

Census Geography in GIS Format

Census geography is available in shape file format from the Census TIGER Download Page. The reports downloaded from Social Explorer may be joined to the geography files based on the Geographic identifier that is common to both datasets.

A Demo Dataset taken from Social Explorer and the TIGER Download Page

The National Historical GIS

The National Historical GIS at the Minnesota population center looks promising -- it seems to have all of the historic data that are in Social Explorer.

Time Series with the Census

Because the census has had digital products for several decades, it is possible to estimate change over time. But it is not easy for several reasons. In 1970 there was no block or block group level data. Over the years, the boundaries of tracts have changed. The questions on the census forms have also changed.

Historic census data can be viewed at a website called Social Explorer. Harvard Affiliates with an ID and PIN can use the social explorer site to download data, as is covered on the web page, Downloading Census Data and Geography.

For an example of a map showing time series census data, Click Here.

Getting Your Data

Easy Census through Geo/ESRI_Data

By far the easiest way to get basic census data at the GSD (if all you need is some very limited population data at the blockgroup or block level) is to use the ESRI Data compilation that can be found in L:\public\geo\esridata_9\usa\census. Even though this collection of data is relatively shallow, it is easy, it has block-level population, and automatically comes with shapefiles!

Downloading Data from the Census Bureau's Web Site or Social Explorer

It is often best to get your data from the primary source. In the case of census data, this is a tad more difficult than getting it from the ESRI Data collection, because you have to understand a lot more about the census (which is not such a bad thing!) and you also have so much more information to choose from -- which can be a little overwhelming. And you also must get the tabular information and geographic features independently and merge them together. This is not so bad, once you get the hang of it. YOu can read and work through a nice tutorial: Getting Census Data and Geography Files

The Census Transportation Planning Package

Click Here to see smoe amazing products derived from the responses to the census questions about the journey to work.