Exercise Bivariate Thematic Map

Download Northwest Alameda County Block Groups.

This is a map layer containing BLOCKGROUPS. Blockgroups are subdivisions of census tracts. Oakland's circa 400,000 population is spread among circa 100 census tracts yielding an average of 4000 people per tract. Blockgroups come in at circa 1500 people. They are further subdivided into CENSUS BLOCKS. As you can see in the diagram below (focus on the sequence down the center - BNA="block numbering areas"), blocks are the smallest unit of analysis in the TIGER/census hierarchy. For a very quick history/overview of tracts, etc. see Tracts and Block Numbering Areas at the census.gov website.


Our layer looks like this:


Attached to these features is age and sex data from the 2010 census.


Across Alameda county about 12.7% of the population is 65 or older (11.1% of men, 14.4% of women). And the overall population is 49.4% male, 50.6% female for a male:female sex ratio of 0.975.

The data has been "recoded" with fields indicating whether the proportion of senior citizens is above the county average (TRUE = more older people than the county average) or not and whether the sex ratio in the block group is below the county average (TRUE = more women than average).

These have been combined in a bivariate index ($=ExtraF \times 2 + ExtraOld$) in which

Code Interpretation
0 Below average women and below average seniors
1 Below average women, above average seniors
2 Above average women, below average seniors
3 Above average women, and above average seniors

Create a thematic map visualizing this information. Write a short commentary in which you discuss any spatial patterns you see, any questions you have about the analysis.


US Census Bureau. Census Tracts