About Mapping with Census Data
The US Census of population and housing is an important resource for understanding the landscape. 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 how GSD students and faculty can make useful maps of census data from 2000 and 4 previous decades.
Understanding Census Data and How to Get It
- Decennial Census Survey
- Census Geography
- Census Time Series
- Getting Census Data
- A Note About Normalization
Related Documents
- Getting Census Data at the GSD
- GIS Data Resources
- Getting Started with ArcGIS at the GSD.
- Normalization: Mapping with Quantitative Data.
The Decennial Census Survey
The US Bureau of the Census is responsible for collecting information used to dertermine allotments of governmental resources (including congressional representatives and education funding.) At the last year of each decade on April 1st, each household of the united states recieves a questionaire (short form) regarding income, ethnicity and housing, every sixth household receives a long form questionaire with very detailed questions including their commute, rent, number of bathrooms, etc. Details about the data published by the census can be found At the Census Web Site
Census Geography
In order to protect the confidentiality of individuals, the bureau releases only summary statistics for geographic areas: blocks, block-groups and tracts.
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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. |
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.
A commercial company, Geolytics has massaged the 1970, 1980 and 1990 and a limited amount of 2000 data in a product called Census CD 40Year. This has reinterpreted 4 decades of census data to the boundaries of the 2000 census.
For an example of a map showing time series census data, Click Here.
Getting Your Data
For many reasons it is best to get your data from the primary source. If you want to download data from the census bureau, you can use the Download Center at Census Bureau's American Factifinder Site. (Click the Download Center) link at the left. Using this tool will get you a text file that has values delimited by a "|" (also known as a Pipe Delimited file. It will open in excel. YOu can get your census geometries from Harvard's Geospatial Library Then you can join the demographic table with the geographic table using a a table join..
A reasonable person might ask why the census bureau doesn't have an easier-to-use web site. But if you cmpare this with the accessibility of census data in other countries, you would be glad to have any sort of access at all without paying a lot of money. ANother thing we can be thankful for is that there is no copyright regulation of this public data. This means that The difficulty of getting census data on line has created a niche for a private company, geolytics to repackage data in a product that makes it easy to get the data you need and connect it with maps. If you are on the GSD network, you can access many Geolytics products directly from your desk. To learn about this, see the manual page named Getting Census Data at the GSD
The Census Transportation Planing Package
Click Here to see smoe amazing products derived from the responses to the census questions about the journey to work.
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.