Draft, July 2015
Background: Community Fact Finder runs demographic summaries for a half mile radius of any point drawn in California. The demographic data is from the Census Bureaus’ American Community 2008-13 5-year survey. To operate, the program draws a half mile circle around the selected point. It then finds all of the block groups that intersect with that radius. The intersected block groups are split so only the portion of the block group polygons that are inside of the polygon are retained. While splitting the block groups, the program assumes and equal distribution of population. Therefore, if only 1/3 of the block group is inside of the radius, only 1/3 of the population is counted. The resulting full and partial block groups are then summarized and reported. While this method produces accurate results in urban areas, rural areas with small pocket populations can be improved. To improve rural area calculations, one needs to distribute the block group data proportionally down to the census block level. This is done to avoid large geographic block groups with a small populated portion from being over distributed. For example, a block group covers 35 square miles with a population of 1,000. Inside that block group there is one contiguous 5 square mile cluster with 900 people, the other 100 people are spread out over the remaining 30 square miles. By breaking the block group geometry down to blocks you can more accurately place the population and reported demographics.
Methods to distribute demographic data to block level:
Data available – American Community Survey 2009-13 5 year estimates at a block group geometry and attributes of interest (population, median household income, and unemployment), 2010 decennial Census block geometry and population
Goal – Distribute the ACS 2009-13 attributes down to the 2010 block geometry.
General outline of methods –
Data sources and notations:
Census demographic data was downloaded from: http://www.census.gov/geo/maps-data/data/tiger-data.html as a file gdb.
American Community Survey (ACS) 2009-2013 5-year estimates, block groups:
Initially 23,212 block groups in California
67 block groups were removed because they had no population: 22 had no land (coastal, water), 45 we reserves, tribal lands, etc.
California population 37,659,181
Census 2010, decennial, blocks
710,145 blocks in California
California population 37,253,956
1 https://www.nhgis.org/user-resources/faq#compare_ACS_and_decennial, cited 4/29/2015 “Which ACS year range is best for comparisons with decennial censuses? For comparisons with older census data, NHGIS highly recommends using an ACS dataset with a year range centered on 2010 (the 2010 1-year data, the 2009-2011 3-year data, or the 2008-2012 5-year data) in order to maintain a consistent decade interval between the "center points" of the study's measurement periods.”
2 http://www.census.gov/acs/www/guidance_for_data_users/comparing_2012/ cited 4/29/2015