CAPS: Circular Area Profiles

Usage Notes

Describing the Updated Version - October, 2005

What The 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). The report is, by default, in the standard format of the MCDC's dp3_2k demographic profile reports. The user may opt to display less data than appears in the full profile report using the Select the Demographic Table(s) of Interest select list.

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:

  • 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.
  • 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 batchgeo.com 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.)
  • To view the area we suggest using Google Maps. This is also our favorite geocoder (hence, the link from the caps page), especially in light of its ability to magically pass back the coordinates to our calling app so that you do not have to do any cut-and-paste.

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 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.
  • 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 use block groups (actually the units are Summary Level 090, or block groups split by MCD, place, CD and urban/rural portion) when the smallest radius is 100 miles or less, and will use county subdivisions (MCDs) otherwise. When block groups are used there are a handful of items that are derived from PCT/HCT tables that are not available at the block group level. Specfically, the last 5 items in Table 8 on the foreign born population, and the last item in Table 23, Grandparents Caring for Own Grandchildren. If these items are critical for you, then you should choose to use MCDs as the geographic unit. (But then you may have problems with geographic rounding for smaller circles. There are a few MCDs in the country, such as the cities of St. Louis, MO and Birmingham, AL that are quite large but are considered single MCDs).
  • 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.

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. For example, when we ran a CAPS report where we selected a large radius of Boston and restricted selection to the 6 New England states (so that the geography selected was exactly all of New England) our report showed a Median Household Income value of $51,092. The value reported on SF3 was $48,382; so our weighted average estimate was off by 5.6%. (All the other counts and averages, we noted, were exact matches with the official sf3 summary, which was reassuring.)

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.

Estimated and Projected Population Estimates

The MCDC has contracted with Easi Analytic Software, Inc of Bellmawr, NJ to allow us to use their recent population estimates and 5-year projections, aggregated to N-mile circular areas. Access is limited to just these aggregate figures; we cannot provide the data for any smaller geographic units. These aggregated totals are displayed on 2 lines between the heading of the report and body of the tables. Users are cautioned to recall that these are NOT official census data, nor even official census estimates. Population projections for small geographic areas are notoriously unreliable. These estimates will be 1 to 2 years old and our plan is to update them each summer. As of July 24, 2006 the estimates are for Jan 1, 2005. We expect to replace these with Jan 1, 2006 estimates in the summer of 2007. The projections, which are currently for Jan 1, 2010, will then be for Jan 1, 2011.


The CAPS application URL is http://mcdc.missouri.edu/websas/caps.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 Glenn Rice at OSEDA.