CAPS: Circular Area Profiles - 2010 Census Version

Usage Notes

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 (census block group or MCD/county subdivision level) that is located within the specified circular areas and aggregates them to create (approximate) circular area summaries. An area (block group or MCD) is selected as being "in" the circle if the coordinates of an internal point for the area (as reported on the Census Bureau's SF3 files) is within the circle. The primary output is a summary report with the demographic characteristics of the circular area(s) based on data collected in the 2010 decennial census. The report is in the standard format of the MCDC's SF1 2010 demographic profile reports.

Specifying the Coordinates

The hardest part of using this application is often determining the coordinates of the site location in degrees of latitude and longitude. An easy alternative, if you do not need great precision as to the exact location, is to just enter a 5-digit ZIP code (a "ZCTA" code, technically, i.e. a ZIP-like entity recognized by the Census Bureau for tabulating the 2000 census) in the box othererwise used for the latitude coordinate. When the program detects a 5-character value with no decimal point it will attempt to do a lookup of the ZCTA internal point coordinates as provided on Summary File 3, and will use those as the values of latitude and longitude. If no site name is specified it will assign this ZCTA code as the value of sitename as well.

When entering coordinate values you can use either decimal degrees (e.g. 37.01234) or you can specify the value in degrees, minutes and seconds using 2 decimal points (e.g. 37.10.15 - indicating 37 degrees, 10 minutes, 15 seconds). Longitudes are assumed to be west and can be entered with or without a leading minus sign.

There are a number of web sites that can assist in determining coordinates for a location. Links are provided here to some of those sites:

  • The Google maps link (via the "here" button) lets you either select your location by navigating a map and placing the cursor over the location, or entering a street addresss (in text box labeld "Lookup Address" at the bottom of the page -- you may need to scroll down to see it) and having the application locate ("geocode") the address. The results (lat-long coordinates) are displayed in text boxes at the bottom of the Google Maps page (which, again, you may need to scroll down to see). In most browsers these days the results are automagically copied into the two lat-long text boxes on the CAPS page, so you do not have to do any copy-and-pasting as you do with all the other methods.

  • This on-line application from Tim Henderson is just what the doctor ordered: you type in an address and it displays the coordinates along with a google street map. If you don't like the location exactly (if you entered an address that was just near the location you really wanted) the app lets you move it and displays the coordinates of the location you specify. (Google Maps can also be used, but is not quite as simple as this app.)

  • The Melissa address lookup web site is one of our favorites for getting the coordinates for any U.S. address. Also returns lots of geocodes, in case you care.

  • The address lookup application is [was] fast and handy for getting the coordinates of an address, along with a map. (The site has been modified and now does really nice Google mapping of one or more addresses, but we were unable to get coordinates for the addresses even though they were clearly geocoded.)

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 Select one or more states.. option lets you restrict the program to selecting geographic 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.

  • The Select the geographic units to be aggregated option can be used (rarely) to override the default value that will normally be chosen by the application. By default CAPS will decide what the best unit to use is based on the smallest radius specified. Unless you have some special reason for overriding this default we suggest you leave this option as specified.

  • 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 group or MCD) that was used to create the circular summary. Be careful when specifying this option for large radii since it can generate a very lengthy report and can sometimes cause a browser to hang when tryingto view.

  • Check to generate txt file with standard codes for all selected geographic areas used in the aggregations - Checking this box results in generating a .txt file where all the geographic areas used to form the circular areas are identified, along with the smallest specified radius containing each area. Look for the link to this file at the bottom of your output page: Use this link to download the geocodes file (all geographic areas used). This file is intrended to be used in a GIS application to display the circular areas. (See, for example, the cv_apps page at Proximty One.)

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 MCDs 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 actually be aggregated using weighted averages where the weight is the denominator of the percent calculation.

The CAPS application URL is

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.