CAPS: Circular Area Profiles - ACS Version

Usage Notes

Rev. 12-21-2016

What This CAPS Application Does

This application lets the user specify a site (point location using lat/long coordinates or by entering a ZIP code) anywhere in the U.S. along with one or more radius values in miles. The application retrieves small-area data (either 2010 block group or census tract, depending on smallest radius) that is located within the specified circular areas and aggregates them to create (approximate) circular area summaries. An area is selected as being "in" the circle if the coordinates of an internal point for the area (as reported on the Census Bureau's 2010 SF1 files) are within the circle. The primary output is a summary report with the aggregated characteristics of the circular area(s) based on data in the Missouri Census Data Center's standard ACS profiles. These data will be taken from the most current 5-year period estimates (2011-2015 as of December, 2016). The report is similar in format and content to that of the MCDC's ACS Profile Reports.

Specifying the Coordinates

The Google maps button on the form lets you either select your location by navigating a map and placing the cursor over the location, or entering a street addresss and having the application locate the address. The results (coordinates) are displayed in text boxes at the bottom of the Google Maps page (which you may need to scroll the window to see). For most modern browsers the results are automatically captured and inserted into the appopriate pair of coordinate text boxes on the form. If that should not work for you then you can copy and paste the coordinate values into the caps form.

Alternatively you can enter a 5-digit ZIP code in the box where you would normally enter the Latitude value. If the program detects a 5-character value, all numeric, then it will do a lookup to get the internal point coordinates of the ZIP code, or ZCTA equivalent, actually. If a non-ZCTA ZIP code is entered we look for a "parent" ZIP code which is also a ZCTA. So if you enter 65211 the program will determine (from the list of alternate ZIPs on the ZCTA Master data set) that ZCTA 65201 can be used instead. The file (data set) used to do the lookup is zcta_master in the georef subdirectory (sometimes referred to as the "ZCTA Master" set). The coordinates in that data set come from a geography file released as part of a recent ACS 5-year data release (as of May, 2014 we are using 2007-2011 data).

Parameters and Options

There are a number of options that allow you to have more control over how your report is generated and what kind of detail you need in your output. Most are somewhat obvious but these notes help you know what to expect.
  • The Enter up to 5 radius values... box is a required input. In an earlier version of this application you were not allowed to enter a radius less than 2; that limitation has been lifted. This coincides with the use of block group level data for smaller circles (< 3 miles). You can enter up to 5 values here and they must be in ascending order, separated by blank spaces. So you can enter 5 10 25 but an entry of 25 5 will be rejected (not in ascending order). Decimal fractions are supported. So you can enter (for example) .75 1.5 3. Keep in mind that just because we allow you to do such small circles does not mean that you should. A 1-mile radius for a rural location is not going to yield very helpful results; it is quite possible in such cases that no block groups will be selected.

    If the program detects a first (smallest) radius value smaller than 3 then it will access the latest block group data; otherwise it will use census tract data (hierarchal level 080: tract within place and county subdivision). If you enter a value of 2 5 10 then bg level data will be used for all 3 of the circles.

  • The Select one or more states.. option lets you restrict the program to selecting geographyic areas within a specified state or states. So if you want a 50-mile radius of Cape Girardeau, MO but do not want data from Illinois you can use this option to specify that you only want Missouri. Normally you do not have to specify what state(s) you want since the program will determine what states intersect your circle.

  • Uncheck to suppress auxiliary report showing county pops within circle(s) - CAPS will generate an auxiliary report following the demographic profiles showing what counties contributed to the data aggregated for each radius. Uncheck this box to suppress this report.

  • Check to see geographic units within circle(s) - This checkbox lets you specify that you want to see a second report (following the CAPS demography summary report) that will list each of the individual geographic units (block groups or tracts) that were used to create the circular summary. Be careful when specifying this option for large radii since it can generate a very lengthy report.

Margins of Error

This application currently does not attempt to calculate MOE's for the aggregated circular data. This is a possible future enhancement. Users need to be aware of the fact that statistical reliability due to a relatively small sampling rate in ACS (vs. the decennial census, for example) can result in significant sampling error. The larger the radius, the less this should be a problem. If block group data are used then the unweighted sample counts of persons and households are reported, along with estimated sampling percentages. For a raius of less than 3 miles the sampling rates are usually less than 10%.

Geographic "Rounding" of Circular Areas

If you are using CAPS for small radii (say, less than 3 miles) you should be aware that there can be significant "geographic rounding" errors. CAPS aggregates block groups or tracts using a method that totally includes or excludes an area from the circle based on a single internal point. If a block group with 1500 people living in it is near the edge of the circle it will be entirely ignored (i.e. its numbers will not be included in the aggregation for the circular area) if the internal point falls just outside the circle, even though in reality many of those 1500 persons do live within the circle. This is a limitation of the available census data. If all you really need is the total population of a circle you can get a better estimate by using smaller geographic units -- census blocks (for which there is no sample data, i.e. nothing about income, poverty, education, etc.) The MABLE/Geocorr geographic utility application has the capability of selecting block level data for a radius and can thus be used to obtain a more accurate population figure.

Averages and Medians

When aggregating the data the program handles averages and medians (as opposed to simple counts) by taking weighted averages. This yields accurate results for averages (such as Average Household Income or Per Capita Income) but not for medians. The nature of medians prohibits their true aggregation unless you can aggregate a distribution from which the median can be derived (or at least estimated) in a post-aggregation step. But no such distributions are used here. To get the aggregated Median Household Income (for example) the CAPS program just takes the weighted average of the medians, weighted by the number of households. This generally provides a pretty good estimate, but it is not precise.

Note that percentage items (such as percent of persons aged 0-4) can be (and are) aggregated using weighted averages where the weight is the denominator of the percent calculation.


The CAPS application URL is http://mcdc.missouri.edu/websas/caps10acs.html.


The   Missouri Census Data Center   is a sponsored program of the Missouri State Library within the office of the Missouri Secretary of State. The MCDC has been a partner in the U.S. Census Bureau's   State Data Center  program since 1979.

Questions and comments can be addressed to John Blodgett at OSEDA.