census geography

Census ACS 2020 and Pop Estimates 2021

Last week, the Census Bureau released the latest 5-year estimates for the American Community Survey for 2016-2020. This latest dataset uses the new 2020 census geography, which means if you’re focused on using the latest data, you can finally move away from the 2010-based geography which had been used for the ACS from 2010 to 2019 (with some caveats: 2020 ZCTAs won’t be utilized until the 2021 ACS, and 2020 PUMAs until 2022). As always, mappers have a choice between the TIGER Line files that depict the precise boundaries, or the generalized cartographic boundary files with smoothed lines and large sections of coastal water bodies removed to depict land areas. The 2016-2020 ACS data is available via data.census.gov and the ACS API.

This release is over 3 months late (compared to normal), and there was some speculation as to whether it would be released at all. The pandemic (chief among several other disruptive events) hampered 2020 decennial census and ACS operations. The 1-year 2020 ACS numbers were released over 2 months later than usual, in late November 2021, and were labeled as an experimental release. Instead of the usual 1,500 plus tables in 40 subject areas for all geographic areas with over 65,000 people, only 54 tables were released for the 50 states plus DC. This release is only available from the experimental tables page and is not being published via data.census.gov.

What happened? The details were published in a working paper, but in summary fewer addresses were sampled and the normal mail out and follow-up procedures were disrupted (pg 8). The overall sample size fell from 3.5 to 2.9 million addresses due to reduced mailing between April and June 2020 (pg 18), and total interviews fell from 2 million to 1.4 million with most of the reductions occurring in spring and summer (pg 18). The overall housing unit response rate for 2020 was 71%, down from 86% in 2019 and 92% in 2018 (pg 20). The response rate for the group quarters population fell from 91% in 2019 to 47% in 2020 (pg 21). Responses were differential, varying by time period (with the lowest rates during the peak pandemic months) and geography. Of the 818 counties that meet the 65k threshold, response rates in some were below 50% (pg 21). The data contained a large degree of non-response bias, where people who did respond to the survey had significantly different social, economic and housing characteristics from those who didn’t. As a consequence of all of this, margins of error for the data increased by 20 to 30% over normal (pg 18).

Thus, 2020 will represent a hole in the ACS estimates series. The Bureau made adjustments to weighting mechanisms to produce the experimental 1-year estimates, but is generally advising policy makers and researchers who normally use this series to choose alternatives: either the 1-year 2019 ACS, or the 5-year 2016-2020 ACS. The Bureau was able to make adjustments to produce satisfactory 5-year estimates to reduce non-response bias, and the 5-year pool of samples is balanced somewhat by having at least 4 years of good data.

The Population Estimates Program has also released its latest series of vintage 2021 estimates for counties and metropolitan areas. This dataset gives us a pretty sharp view of how the pandemic affected the nation’s population. Approximately 73% of all counties experienced natural decrease in 2021 (between July 1st 2020 and 2021), where the number of deaths outnumbered births. In contrast, 56% of counties had natural decrease in 2020 and 46% in 2019. Declining birth rates and increasing death rates are long term trends, but COVID-19 magnified them, given the large number of excess deaths on one hand and families postponing child birth due to the virus on the other hand. Net foreign migration continued its years-long decline, but net domestic migration increased in a number of places, reflecting pandemic moves. Medium to small counties benefited most, as did large counties in the Sunbelt and Mountain West. The biggest losers in overall population were counties in California (Los Angeles, San Francisco, and Alameda), Cook County (Chicago), and the counties that constitute the boroughs of NYC.

Census Bureau 2021 Population Estimates Map
2020 Resident Population Map

2020 Census Updates

In late summer and early fall I was hammering out the draft for an ALA Tech Report on using census data for research (slated for release early 2022). The earliest 2020 census figures have been released and there are several issues surrounding this, so I’ll provide a summary of what’s happening here. Throughout this post I link to Census Bureau data sources, news bulletins, and summaries of trends, as well as analysis on population trends from Bill Frey at Brookings and reporting from Hansi Lo Wang and his colleagues at NPR.

Count Result and Reapportionment Numbers

The re-apportionment results were released back in April 2020, which provided the population totals for the US and each of the states that are used to reallocate seats in Congress. This data is typically released at the end of December of the census year, but the COVID-19 pandemic and political interference in census operations disrupted the count and pushed all the deadlines back.

Despite these disruptions, the good news is that the self-response rate, which is the percentage of households who submit the form on their own without any prompting from the Census Bureau, was 67%, which is on par with the 2010 census. This was the first decennial census where the form could be submitted online, and of the self-responders 80% chose to submit via the internet as opposed to paper or telephone. Ultimately, the Bureau said it reached over 99% of all addresses in its master address file through self-response and non-response follow-ups.

The bad news is that the rate of non-response to individual questions was much higher in 2020 than in 2010. Non-responses ranged from a low of 0.52% for the total population count to a high of 5.95% for age or date of birth. This means that a higher percentage of data will have to be imputed, but this time around the Bureau will rely more on administrative records to fill the gaps. They have transparently posted all of the data about non-response for researchers to scrutinize.

The apportionment results showed that the population of the US grew from approximately 309 million in 2010 to 331 million in 2020, a growth rate of 7.35%. This is the lowest rate of population growth since the 1940 census that followed the Great Depression. Three states lost population (West Virginia, Mississippi, and Illinois), which is the highest number since the 1980 census. The US territory of Puerto Rico lost almost twelve percent of its population. Population growth continues to be stronger in the West and South relative to the Northeast and Midwest, and the fastest growing states are in the Mountain West.

https://www.census.gov/library/visualizations/2021/dec/2020-percent-change-map.html

Public Redistricting Data

The first detailed population statistics were released as part of the redistricting data file, PL 94-171. Data in this series is published down to the block level, the smallest geography available, so that states can redraw congressional and other voting districts based on population change. Normally released at the end of March, this data was released in August 2021. This is a small package that contains the following six tables:

  • P1. Race (includes total population count)
  • P2. Hispanic or Latino, and Not Hispanic or Latino by Race
  • P3. Race for the Population 18 Years and Over
  • P4. Hispanic or Latino, and Not Hispanic or Latino by Race for the Population 18 Years and
    Over
  • P5. Group Quarters Population by Major Group Quarters Type
  • H1. Occupancy Status (includes total housing units)

The raw data files for each state can be downloaded from the 2020 PL 94-171 page and loaded into stats packages or databases. That page also provides infographics (including the maps embedded in this post) and data summaries. Data tables can be readily accessed via data.census.gov, or via IPUMS NHGIS.

The redistricting files illustrate the increasing diversity of the United States. The number of people identifying as two or more races has grown from 2.9% of the total population in 2010 to 10.2% in 2020. Hispanics and Latinos continue to be the fastest growing population group, followed by Asians. The White population actually shrank for the first time in the nation’s history, but as NPR reporter Hansi-Lo Wang and his colleagues illustrate this interpretation depends on how one measures race; as race alone (people of a single race) or persons of any race (who selected white and another race), and whether or not Hispanic-whites are included with non-Hispanic whites (as Hispanic / Latino is not a race, but is counted separately as an ethnicity, and most Hispanics identify their race as White or Other). The Census Bureau has also provided summaries using the different definitions. Other findings: the nation is becoming progressively older, and urban areas outpaced rural ones in population growth. Half of the counties in the US lost population between 2010 and 2020, mostly in rural areas.

https://www.census.gov/library/visualizations/2021/dec/percent-change-county-population.html

2020 Demographic and Housing Characteristics and the ACS

There still isn’t a published timeline for the release of the full results in the Demographic and Housing Characteristics File (DHC – known as Summary File 1 in previous censuses, I’m not sure if the DHC moniker is replacing the SF1 title or not). There are hints that this file is going to be much smaller in terms of the number of tables, and more limited in geographic detail compared to the 2010 census. Over the past few years there’s been a lot of discussion about the new differential privacy mechanisms, which will be used to inject noise into the data. The Census Bureau deemed this necessary for protecting people’s privacy, as increased computing power and access to third party datasets have made it possible to reverse engineer the summary census data to generate information on individuals.

What has not been as widely discussed is that many tables will simply not be published, or will only be summarized down to the county-level, also for the purpose of protecting privacy. The Census Bureau has invited the public to provide feedback on the new products and has published a spreadsheet crosswalking products from 2010 and 2020. IPUMS also released a preliminary list of tables that could be cut or reduced in specificity (derived from the crosswalk), which I’m republishing at the bottom of this post. This is still preliminary, but if all these changes are made it would drastically reduce the scope and specificity of the decennial census.

And then… there is the 2020 American Community Survey. Due to COVID-19 the response rates to the ACS were one-third lower than normal. As such, the sample is not large or reliable enough to publish the 1-year estimate data, which is typically released in September. Instead, the Census will publish a smaller series of experimental tables for a more limited range of geographies at the end of November 2021. There is still no news regarding what will happen with the 5-year estimate series that is typically released in December.

Needless to say, there’s no shortage of uncertainty regarding census data in 2020.

Tables in 2010 Summary File 1 that Would Have Less Geographic Detail in 2020 (Proposed)

Table NameProposed 2020 Lowest Level of Geography2010 Lowest Level of Geography
Hispanic or Latino Origin of Householder by Race of HouseholderCountyBlock
Household Size by Household Type by Presence of Own ChildrenCountyBlock
Household Type by Age of HouseholderCountyBlock
Households by Presence of People 60 Years and Over by Household TypeCountyBlock
Households by Presence of People 60 Years and Over, Household Size, and Household TypeCountyBlock
Households by Presence of People 75 Years and Over, Household Size, and Household TypeCountyBlock
Household Type by Household SizeCountyBlock
Household Type by Household Size by Race of HouseholderCountyBlock
Relationship by Age for the Population Under 18 YearsCountyBlock
Household Type by Relationship for the Population 65 Years and OverCountyBlock
Household Type by Relationship for the Population 65 Years and Over by RaceCountyBlock
Family Type by Presence and Age of Own ChildrenCountyBlock
Family Type by Presence and Age of Own Children by Race of HouseholderCountyBlock
Age of Grandchildren Under 18 Years Living with A Grandparent HouseholderCountyBlock
Household Type by Relationship by RaceCountyBlock
Average Household Size by AgeTo be determinedBlock
Household Type for the Population in HouseholdsTo be determinedBlock
Household Type by Relationship for the Population Under 18 YearsTo be determinedBlock
Population in Families by AgeTo be determinedBlock
Average Family Size by AgeTo be determinedBlock
Family Type and Age for Own Children Under 18 YearsTo be determinedBlock
Total Population in Occupied Housing Units by TenureTo be determinedBlock
Average Household Size of Occupied Housing Units by TenureTo be determinedBlock
Sex by Age for the Population in HouseholdsCountyTract
Sex by Age for the Population in Households by RaceCountyTract
Presence of Multigenerational HouseholdsCountyTract
Presence of Multigenerational Households by Race of HouseholderCountyTract
Coupled Households by TypeCountyTract
Nonfamily Households by Sex of Householder by Living Alone by Age of HouseholderCountyTract
Group Quarters Population by Sex by Age by Group Quarters TypeStateTract

Tables in 2010 Summary File 1 That Would Be Eliminated in 2020 (Proposed)

Population in Households by Age by Race of Householder
Average Household Size by Age by Race of Householder
Households by Age of Householder by Household Type by Presence of Related Children
Households by Presence of Nonrelatives
Household Type by Relationship for the Population Under 18 Years by Race
Household Type for the Population Under 18 Years in Households (Excluding Householders, Spouses, and Unmarried Partners)
Families*
Families by Race of Householder*
Population in Families by Age by Race of Householder
Average Family Size by Age by Race of Householder
Family Type by Presence and Age of Related Children
Family Type by Presence and Age of Related Children by Race of Householder
Group Quarters Population by Major Group Quarters Type*
Population Substituted
Allocation of Population Items
Allocation of Race
Allocation of Hispanic or Latino Origin
Allocation of Sex
Allocation of Age
Allocation of Relationship
Allocation of Population Items for the Population in Group Quarters
American Indian and Alaska Native Alone with One Tribe Reported for Selected Tribes
American Indian and Alaska Native Alone with One or More Tribes Reported for Selected Tribes
American Indian and Alaska Native Alone or in Combination with One or More Other Races and with One or More Tribes Reported for Selected Tribes
American Indian and Alaska Native Alone or in Combination with One or More Other Races
Asian Alone with One Asian Category for Selected Groups
Asian Alone with One or More Asian Categories for Selected Groups
Asian Alone or in Combination with One or More Other Races, and with One or More Asian Categories for Selected Groups
Native Hawaiian and Other Pacific Islander Alone with One Native Hawaiian and Other Pacific Islander Category for Selected Groups
Native Hawaiian and Other Pacific Islander Alone with One or More Native Hawaiian and Other Pacific Islander Categories for Selected Groups
Native Hawaiian and Other Pacific Islander Alone or in Combination with One or More Races, and with One or More Native Hawaiian and Other Pacific Islander Categories for Selected Groups
Hispanic or Latino by Specific Origin
Sex by Single Year of Age by Race
Household Type by Number of Children Under 18 (Excluding Householders, Spouses, and Unmarried Partners)
Presence of Unmarried Partner of Householder by Household Type for the Population Under 18 Years in Households (Excluding Householders, Spouses, and Unmarried Partners)
Nonrelatives by Household Type
Nonrelatives by Household Type by Race
Group Quarters Population by Major Group Quarters Type by Race
Group Quarters Population by Sex by Major Group Quarters Type for the Population 18 Years and Over by Race
Total Races Tallied for Householders
Hispanic or Latino Origin of Householders by Total Races Tallied
Total Population in Occupied Housing Units by Tenure by Race of Householder
Average Household Size of Occupied Housing Units by Tenure
Average Household Size of Occupied Housing Units by Tenure by Race of Householder
Occupied Housing Units Substituted
Allocation of Vacancy Status
Allocation of Tenure
Tenure by Presence and Age of Related Children
* Counts for these tables are available in other proposed DHC tables. For example, the count of families is available in the Household Type table, which will be available at the block level in the 2020 DHC. 
Census Tracts

Call for Proposals: Celebrating the Census in the Journal of Maps

I’m serving as a co-editor for a special issue for the Journal of Maps entitled “Celebrating the Census“. The Journal of Maps is an open access, peer reviewed journal published by the Taylor & Francis Group. The journal is distinct in that all articles feature maps and spatial diagrams as the focal point for studying geographic phenomena from both a physical / environmental and social science perspective.

Here’s the official synopsis for this census-themed special issue:

We invite contributions to a special issue of the Journal of Maps focused upon the evolving character and cartographic opportunities offered by traditional census statistics and the impact of transitioning from these sources of population data at a range of spatial scales into a new era of big data assembly. In so doing, the special issue marks two important events taking place in the UK during 2021 in the history of British Censuses and seeks contributions that reflect the past transition of population data cartography through the digital era of the last 50 years and anticipates its transformation into the big data era of the foreseeable future.

While the issue marks the 100th anniversary of the UK census, submissions concerning census mapping from around the world are welcome and encouraged in these topic areas, including but not limited to:

  • Spatial and statistical consistency over time
  • People on the move
  • Mapping people through space and time
  • Mapping morbidity and mortality
  • Politics and population data
  • International comparison of demographic mapping
  • Before and after population mapping using censuses and administrative sources
  • Population data and mapping human-environmental interaction
  • Transition and evolution in population mapping

Visit the special issue announcement for full details. Deadlines:

  • April 30, 2021: a short draft (500-word limit) outlining themes and scope of the paper, preferably with a sample map
  • June 14, 2021: abstracts will be selected by the editorial team by this date
  • Sept 5, 2021: completed paper (4000-word limit) is due

The issue will be published sometime in 2022.

Stamen Watercolor Map Tiles

Adding Basemaps to QGIS With Web Mapping Services

For this final post of 2020, I was looking back through recent projects for something interesting yet brief; I’ve been writing some encyclopedia-length posts lately and wanted to keep this one on the lighter side. In that vein, I’ve decided to share a short list of free web mapping services that I use as basemaps in QGIS (they’ll work in ArcGIS too). This has been on my mind as I’ve recently stumbled upon the OpenTopoMap, which is an alternate stylized version of the OpenStreetMap that looks pretty sharp.

See this earlier post for details, but in short, to connect to these services in QGIS:

QGIS Browser Panel
  1. Select the appropriate web map service type in the browser panel (usually WMS / WMTS or XYZ Tiles), right click, and add new connection.
  2. Give it a meaningful name, paste the appropriate URL into the URL box, click OK.
  3. In the browser panel drill down to see the service, and for WMS / WMTS layers you can drill down further to see specific layers you can add.
  4. Select the layer and drag it into the window, or select, right click, and add the layer to the project.
  5. If the resolution looks off, right click on a blank area of the toolbar and check the Tile Scale Panel. Use this to adjust the zoom for the web map. If the scale bar is greyed out you’ll need to set the map window to the same CRS as the map service: select the layer in the panel, right click, and choose set CRS – set project CRS from layer.
  6. Some web layers may render slowly if you’re zoomed out to the full extent, or even not at all if they contain many features or are super detailed. Conversely, some layers may not render if you’re zoomed too far in, as tiles may not be available at that resolution. Experiment!

If you’re an ArcGIS user see these concise instructions for adding various tile layers. This isn’t something that I’ve ever done, as ArcGIS already has a number of accessible basemaps that you can add.

In the list below, links for the service name take you to either the website version of the service, or to a list of additional layers that you can connect to. The URLs that follow are the actual connections to the service that you’ll use within your GIS package. If you use OSM, OTP, or Stamen in your maps, make sure to cite them (they use Creative Commons Licenses – follow links to their websites for details). The government sources are public domain, but you should still cite them anyway. Happy mapping, and happy holidays!

OpenStreetMap XYZ Tile (global)

http://tile.openstreetmap.org/{z}/{x}/{y}.png

OpenTopoMap XYZ Tile (global)

https://tile.opentopomap.org/{z}/{x}/{y}.png

Stamen XYZ Tile (global) see their website for examples; the image topping this post is from watercolor

http://tile.stamen.com/terrain/{z}/{x}/{y}.png
http://tile.stamen.com/toner/{z}/{x}/{y}.png
http://tile.stamen.com/watercolor/{z}/{x}/{y}.jpg

USGS National Map WMTS (global, but fine detail is US only)

Imagery:
https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/WMTS/1.0.0/WMTSCapabilities.xml

Imagery & Topo:
https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryTopo/MapServer/WMTS/1.0.0/WMTSCapabilities.xml

Shaded Relief: 
https://basemap.nationalmap.gov/arcgis/rest/services/USGSShadedReliefOnly/MapServer/WMTS/1.0.0/WMTSCapabilities.xml

Topographic:
https://basemap.nationalmap.gov/arcgis/rest/services/USGSTopo/MapServer/WMTS/1.0.0/WMTSCapabilities.xml

US Census Bureau TIGERweb WMS (US only) see their website for older vintages

Current TIGER features:
https://tigerweb.geo.census.gov/arcgis/services/TIGERweb/tigerWMS_Current/MapServer/WMSServer 

Current physical features:
https://tigerweb.geo.census.gov/arcgis/services/TIGERweb/tigerWMS_PhysicalFeatures/MapServer/WMSServer

atcoordinates YouTube Channel

Video Tutorials for Finding US Census Data

I have recently created an atcoordinates YouTube channel that features a series of how-to videos on finding and accessing US census data using a variety of websites and tools. I explain basic census concepts while demonstrating how to access data. At this point there are four videos:

  1. Exploring US Census Data: Basic Concepts. This is a narrated slide show where I cover the essential choices you need to make and concepts you need to understand in order to access census data, regardless of the tool or platform: data set, time period, subjects or topics, and geography. I discuss the decennial census, American Community Survey, and population estimates. This video is intended as a prerequisite for viewing the others, so I don’t have to explain the same concepts each time and can focus on demonstrating each particular application.
  2. American Community Survey Census Profiles with MCDC Apps. This screencast illustrates how you can quickly and easily access census profiles for any place in the US using the Missouri Census Data Center’s profile applications. It’s also a good introduction to census data in general, if you’re unfamiliar with the scope of data that’s available.
  3. Search Strategies for data.census.gov. I demonstrate how to use the Census Bureau’s primary application for accessing current census data, using the advanced search tool and filters.
  4. Using TIGERweb to Explore US Census Geography. I show you how to use this web map application for viewing census geography, while explaining what some of the small-area census geographies are.

I plan on adding additional videos every month or so. The pandemic lock down and uncertainty over whether classes will be back in session this fall inspired me to do this. While I prefer written tutorials, I find that I’ve been watching YouTube more often for learning how to do certain tasks with particular software, so I thought this would be useful for others. The videos average about 10 to 15 minutes in length, although the introductory one is a bit longer. The length is intentional; I wanted to explain the concepts and describe why you’re making certain choices, instead of simply pointing and clicking without any explanation.

Feel free to spread the news, share and embed the videos in research guides or web pages, and use them in classes or workshops. Of course, for a more in-depth look at US census data, check out my book: Exploring the US Census: Your Guide to America’s Data published by SAGE.

USPS mailbox

The Trouble with ZIP Codes: Solutions for Data Analysis and Mapping

Since the COVID-19 pandemic began, I’ve received several questions about finding census data and boundary files for ZIP Codes (aka US postal codes), as many states are publishing ZIP Code-level data for cases and deaths. ZIP Codes are commonly used for summarizing address data, as it’s easy to do and most Americans are familiar with them. However, there are a number of challenges associated with using ZIP Codes as a unit of analysis that most people are unaware of (until they start using them). In this post I’ll summarize these challenges and provide some solutions.

The short story is: you can get boundary files and census data from the decennial census and 5-year American Community Survey (ACS) for ZIP Code Tabulation Areas (ZCTAs, pronounced zicktas) which are approximations of ZIP Codes that have delivery areas. Use any census data provider to get ZCTA data: data.census.gov, Census Reporter, Missouri Census Data Center, NHGIS, or proprietary library databases like PolicyMap or the Social Explorer. The longer story: if you’re trying to associate ZIP Code-level data with census ZCTA boundary files or demographic data, there are caveats. I’ll cover the following issues in detail:

  1. ZIP Codes are actually not areas with defined boundaries, and there are no official USPS ZIP Code maps. Areas must be derived using address files. The Census Bureau has done this in creating ZIP Code Tabulation Areas (ZCTAs).
  2. The Census Bureau publishes population data by ZCTA and boundary files for them. But ZCTAs are not strictly analogous with ZIP Codes; there isn’t a ZCTA for every ZIP Code, and if you try to associate ZIP data with them some of your records won’t match. You need to crosswalk your ZIP Code data to the ZCTA-level to prevent this.
  3. ZCTAs do not nest or fit within any other census geographies, and the postal city name associated with a ZIP Code does not correlate with actual legal or municipal areas. This can make selecting and downloading ZIP Code data for a given area difficult.
  4. ZIP Codes were designed for delivering mail, not for studying populations. They vary tremendously in size, shape, and population.
  5. Analyzing data at either the ZIP Code or ZCTA level over time is difficult to impossible.
  6. ZIP Code and ZCTA numbers must be saved as text in data files, and not as numbers. Otherwise codes that have leading zeros get truncated, and the code becomes incorrect.

ZIP Codes versus ZCTAs and Boundaries

Contrary to popular belief, ZIP Codes are not areas and the US Postal Service does not delineate boundaries for them. They are simply numbers assigned to ranges of addresses along street segments, and the codes are associated with a specific post office. When we see ZIP Code boundaries (on Google Maps for example), these have been derived by creating areas where most addresses share the same ZIP Code.

The US Census Bureau creates areal approximations for ZIP Codes called ZIP Code Tabulation Areas or ZCTAs. The Bureau assigns census blocks to a ZIP number based on the ZIP that’s used by a majority of the addresses within each block, and aggregates blocks that share the same ZIP to form a ZCTA. After this initial assignment, they make some modifications to aggregate or eliminate orphaned blocks that share the same ZIP number but are not contiguous. ZCTAs are delineated once every ten years in conjunction with the decennial census, and data from the decennial census and the 5-year American Community Survey (ACS) are published at the ZCTA-level. You can download ZCTA boundaries from the TIGER / Line Shapefiles page, and there is also a generalized cartographic boundary file for them.

Crosswalking ZIP Code Data to ZCTAs

There isn’t a ZCTA for every ZIP Code. Some ZIP Codes represent large clusters of Post Office boxes or are assigned to large organizations that process lots of mail. As census blocks are aggregated into ZCTAs based on the predominate ZIP Code for addresses within the block, these non-areal ZIPs fall out of the equation and we’re left with ZCTAs that approximate ZIP Codes for delivery areas.

As a result, if you’re trying to match either your own summarized address data or sources that use ZIP Codes as the summary level (such as the Census Bureau’s Business Patterns and Economic Census datasets), some ZIP Codes will not have a matching ZCTA and will fall out of your dataset.

To prevent this from happening, you can aggregate your ZIP Code data to ZCTAs prior to joining it to boundary files or other datasets. The UDS Mapper project publishes a ZIP Code to ZCTA Crosswalk file that lists every ZIP Code and the ZCTA it is associated with. For the ZIP Codes that don’t have a corresponding area (the PO Box clusters and large organizations), these essentially represent points that fall within ZCTA polygons. Join your ZIP-level data to the ZIP Code ID in the crosswalk file, and then group or summarize the data using the ZCTA number in the crosswalk. Then you can match this ZCTA-summarized data to boundaries or census demographic data at the ZCTA-level.

ZIP Code to ZCTA Crosswalk

UDS ZIP Code to ZCTA Crosswalk. ZIP Code 99501 is an areal ZIP Code with a corresponding ZCTA number, 99501. ZIP Code 99520 is a post office or large volume customer that falls inside ZCTA 99501, and thus is assigned to that ZCTA.

Identifying ZIPs and ZCTAs within Other Areas

ZCTAs are built from census blocks and nest within the United States; they do not fit within any other geographies like cities and towns, counties, or even states. The boundaries of a ZCTA will often cross these other boundaries, so for example a ZCTA may fall within two or three different counties. This makes it challenging to select and download census data for all ZCTAs in a given area.

You can get lists of ZIP Codes for places, for example by using the MCDC’s ZIP Code Lookup. The problem is, the postal city that appears in addresses and is affiliated with a ZIP Code does not correspond with cities as actual legal entities, so you can’t count on the name to select all ZIPs within a specific place. For example, my hometown of Claymont, Delaware has its own ZIP Code, even though Claymont is not an incorporated city with formal, legal boundaries. Most of the ZIP Codes around Claymont are affiliated with Wilmington as a place, even though they largely cover suburbs outside the City of Wilmington; the four ZIP Codes that do cover the city cross the city boundary and include outside areas. In short, if you select all the ZIP Codes that have Wilmington, DE as their place name, they actually cover an area that’s much larger than the City of Wilmington. The Census Bureau does not associate ZCTAs with place names.

ZCTAs and Places in northern Delaware

Lack of correspondence between postal city names and actual city boundaries. Most ZCTAs with the prefix 198 are assigned to Wilmington as a place name, even though many are partially or fully outside the city.

So how can you determine which ZIP Codes fall within a certain area? Or how they do (or don’t) intersect with other areas? You can overlay and eyeball the areas in TIGERweb to get a quick idea. For something more detailed, here are three options:

  1. The Missouri Census Data Center’s Geocorr application lets you calculate overlap between a source geography and a target geography using either total population or land area for any census geographies. So in a given state, if you select ZCTAs as a source, and counties as the target, you’ll get a list that displays every ZCTA that falls wholly or partially within each county. An allocation factor indicates the percentage of the ZCTA (population or land) that’s inside and outside a county, and you can make decisions as to whether to include a given ZCTA in your study area or not. If a ZCTA falls wholly inside one county, there will be only one record with an allocation factor of 1. If it intersects more than one county, there will be a record with an allocation factor for each county.
  2. The US Department of Housing and Urban Development (HUD) publishes a series of ZIP Code crosswalk files that associates ZIP Codes with census tracts, counties, CBSAs (metropolitan areas), and congressional districts. They create these files by geocoding all addresses and calculating the ratio of residential, business, and other addresses that fall within each of these areas and that share the same ZIP Code. The files are updated quarterly. You can use them to select, assign, or apportion ZIP Codes to a given area. There’s a journal article that describes this resource in detail.
  3. Some websites allow you to select all ZCTAs that fall within a given geography when downloading data, essentially by selecting all ZCTAs that are fully or partially within the area. The Census Reporter allows you to do this: search for a profile for an area, click on a table of interest, and then subdivide the areas by smaller areas. You can even look at a map to see what’s been selected. data.census.gov currently does not provide this option; you have to select ZCTAs one by one (or if you’re using the census API, you’ll need to create a list of ZCTAs to retrieve).

MCDC Geocorr

Sample output from MCDC Geocorr. ZCTAs 08251 and 08260 fall completely within Cape May County, NJ. ZCTA 08270’s population is split between Cape May (92.4%) and Atlantic (7.6%) counties. The ZCTA names are actually postal place names; these ZCTAs cover areas that are larger than these places.

Do You Really Need to Use ZIP Codes?

ZIP Codes were an excellent mid-20th century solution for efficiently processing and delivering mail that continues to be useful for that purpose. They are less ideal for studying populations or other forms of human activity. They vary tremendously in size, shape, and population which makes them inconsistent as a unit of analysis. They have no legal or administrative meaning or function, other than delivering mail. While all American’s are familiar with them, they do not have any relevant social meaning. They don’t represent neighborhoods, and when you ask someone where they’re from, they won’t say “19703”.

So what are your other options?

  1. If you don’t have to use ZIP Code or ZCTA data for your project, don’t. For the United States as a whole, consider using counties, PUMAs, or metropolitan areas. Within states: counties, PUMAs, and county subdivisions. For smaller areas: municipalities, census tracts, or aggregates of census tracts.
  2. If you have the raw, address-based data, consider geocoding it. Once you geocode an address, you can use GIS to assign it to any type of geography that you have a boundary file for (spatial join), and then you can aggregate it to that geography. Some geocoders even provide geographies like counties or tracts in the match result. If your data is sensitive, strip all the attributes out except for the address and a serial integer to use as an ID, and after geocoding you can associate the results back to your original data using that ID. The Census Geocoder is free, requires no log in, allows you to do batches of 1,000 addresses at a time, and forces you to use these safety precautions. For bigger jobs, there’s an API.
  3. Sometimes you’ll have no choice and must use ZIP Code / ZCTA data, if what you’re interested in studying is only provided in that summary form, or if there are privacy concerns around geocoding the raw address data. You may want to modify the ZCTA geography for your area to aggregate smaller ZCTAs into larger ones surrounding them, for both visual display and statistical analysis. For example, in New York City there are several ZCTAs that cover only one city and census block, as they’re occupied by one large office building that processes a lot of mail (and thus have their own ZIP number). Also, unlike most census geographies, ZCTAs have large holes in them. Any area that does not have streets and thus no addresses isn’t included in a ZCTA. In urban areas, this means large parks and cemeteries. In rural areas, vast tracts of unpopulated forest, desert, or mountain terrain. And large bodies of water in every place.

Midtown ZCTAs

One-block ZCTAs in Midtown Manhattan, NYC that have either low or zero population.

Analyzing ZIP Code Data Over Time…

In short – forget it. The Census Bureau introduced ZCTAs in the year 2000, and in 2010 they modified their process for creating them. For a variety of reasons, they’re not strictly compatible. ACS data for ZCTAs wasn’t published until 2013. Even the economic datasets don’t go that far back; the ZIP Code Business Patterns didn’t appear until the early 1990s. Use areas that have more longevity and are relatively stable: counties, census tracts.

Why Do my ZIP Codes Look Wrong in Excel?

Regardless of whether you’re using a spreadsheet, database, or scripting language, always make sure to define ZIP / ZCTA columns as strings or text, and not as numeric types. ZIP Codes and ZCTAs begin with zeros in several states. Columns that contain ZIP / ZCTA codes must be saved as text to preserve the 5-digit code. If they’re saved as numbers, the leading zeros are dropped and the numbers are rendered incorrectly. This often happens if you’re working with data in a CSV file and you click on it to open it in Excel. In parsing the CSV, Excel assumes the ZIP / ZCTA field is a number and saves it as a number, which drops the zero and truncates the code. To prevent this from happening: open Excel to a blank project, go to the Data ribbon, click the button to import text data, choose delimited text on the import screen, choose the delimiter (comma or tab, etc), and when prompted you can select the ZIP / ZCTA column and designate it as text so that it imports properly.

Importing text files in Excel

To import CSV files in Excel, go to the Data ribbon and under Get External Data select From Text.

Conclusion

That’s all you ever (or maybe never) wanted to know about ZIP Codes and ZCTAs! For more information see the Census Bureau’s page about ZCTAs, a thorough write up by the Missouri Census Data Center, and these informative and fun blog posts from PolicyMap (complete with photos of Mr. ZIP). I wrote an article a few years back that demonstrates how to use some of these resources (the UDS mapper file, Geocorr) to process ZIP data with SQL and python. And of course, check out my book, Exploring the U.S. Census: Your Guide to America’s Data, to explore these concepts and resources in greater detail with hands-on exercises.