2020 Census: Defining Census Tracts and Boundary Changes

One of the main reasons an accurate Decennial Census count is so important is because of its impact on redistricting and funding, which are determined and impacted by geographic boundaries called Census tracts. In this blog, we will look at how Census boundaries are defined, how the boundaries can be changed, and why these changes matter. 

Need a crash course in the 2020 Census? Check out our 2020 Census introduction blog! 

Why do boundary changes matter?

The Census collects demographic information for a defined geographic area. It asks questions like how many people are living in a house; age, sex, and race/ethnicity of the resident(s); and so on. The data is then summarized for a particular area at every geographic level in the standard hierarchy of census geographic entities, from bottom to top, which includes census blocks, census block groups, census tracts, counties, states, divisions, regions, and the nation.

Diagram showing the hierarchy of census geographies

The goal is to “count everyone once, only once, and in the right place” to help reveal the up-to-date counts of population and housing units, as well as the social and economic trends in any geographic area. 

Since each geographic area serves as a framework for understanding the Census data, boundary changes can raise an early indication of significant demographic changes. For instance, the boundaries of geographies that are defined by the Census Bureau for statistical purposes, like that of census tracts and census blocks, are especially sensitive to population changes: a tract or block may be split into two when the population within has increased beyond a key threshold, or the geographies that have population decreased below a key threshold will be combined into one. 

Population changes within an area impact many decisions, such as the distribution of hundreds of billions of dollars in federal funding, the redistricting of legislative districts, business investments, and community planning. Altogether, these decisions will shape many different aspects of every community until the release of the next Decennial Census. Thus, knowing geographic changes always matters to residents, community leaders, and urban analysts. 

As Metro Detroit’s community data hub, D3 pays close attention to these boundary changes to provide the most accurate portrayal of Detroit communities using various urban data to help with evidence-based community planning decisions. For instance, our data portal State of the Detroit Child allows users to easily find out their community facts derived from the Census by census block, census tract, zip code, city, county, etc., with topics including but not limited to youth & family population, household size, sex, age, race, and ethnicity. 

When Census boundaries change, as they did in the 2020 Census, D3 works diligently to translate and trace these changes from their geographies in the 2010 Census to the new geographies of the 2020 Census. This is vital for community members and organizations who are interested in evaluating trend data by comparing year over year data.

Want to know more about how the Census impacts the distribution of federal funding and redistricting? Check out our Census Series: 

What’s a Census Tract and why does it matter?

Now we know that boundary changes are important, but why do we particularly focus on census tracts? Let’s quickly review the definition of the “Census Tract.”

A Census Tract is a relatively small geographic area—a subdivision of a county—that is established by the Census Bureau for collecting census data. It can be roughly regarded as a neighborhood, as it generally has 2,500 to 8,000 residents and its boundary follows visible features like roads. 

Census tract-level data are incredibly useful for our work. Many useful urban datasets are collected and summarized at the census tract level, including but not limited to, population characteristics, school enrollment, poverty level, median household income, housing vacancy, etc. 

However, census tract boundaries are only “relatively” permanent. They might change based on the Census results for two reasons:

  1. When a census tract’s internal population grows over 8,000 persons, it may split into two or more smaller census tracts.
  2. When two or more adjacent census tracts experience drastic population decline, they may be combined into one new census tract. 

This review and revision process is conducted every decade with collaboration from local planning agencies. And the changes are recorded in the “Relationship Files” released by the Census Bureau.

How do we track census tract boundary changes?

D3 is taking the lead in keeping track of census tract changes in Michigan right after the release of the 2020 Census, to make sure users have access to the most up-to-date community facts at the census tract level, and when users are comparing the Census over the years, they are comparing the data in the same geography. 

We have established three different types of census tract changes during our process:

Split census tracts are new tracts that are created by splitting a single census tract into two new geographies. As mentioned earlier, this type of tract change is usually caused by a significant increase in population. 

These tracts typically retain their original tract ID number, but with additional numbers appended to the end. For instance, the 2010 census tract 26163531500 has been split to form tracts 26163531501 and 26163531502. We’ll be examining tract ID numbers in more detail a bit later in this blog.

Combined census tracts are new census tracts that are formed by merging two or more established census tracts into single geography. These changes are typically made due to a significant decrease in population across neighboring tracts. 

These tracts are typically assigned a new tract ID number by the U.S. Census Bureau but are considered distinct from other “new” census tracts.

New census tracts are created by merging only portions of established Census tracts into new shapes. These might be the result of a shift in population that is disproportionate across the original geography. 

Like combined census tracts, new census tracts are typically assigned new tract ID numbers by the U.S. Census Bureau. 

We identified these census tract changes based on three approaches: relying on the Census Bureau’s renumbering system for geographic changes, visually comparing the boundary shape changes between 2010 and 2020 census tracts, and using GIS tools to compare the geometric differences between 2010 and 2020 census tracts.

Here are some details for each of these approaches:

The Census Bureau’s Numbering System

For efficient data processing and publication, the Census Bureau identifies census tracts by numbers rather than names. Each census tract is assigned with a 4-digit basic number, sometimes with an additional 2-digit suffix, as its unique identifier. This is also called the census tract ID

We were able to easily identify newly created census tracts by comparing census tract IDs from 2010 and 2020. We just needed to pick out IDs from 2020 that were not present among the 2010 census tracts.  

The 2-digit suffix referenced above is used to record modifications that have been made to the census tract throughout the years, such as census tract splits. When new census tracts are created by splitting an old census tract, The U.S. Census Bureau recommends retaining the original tract ID and appending two digits to the end that begin with 01. 

For instance, if there was a census tract “10100” that was split into two new tracts in the 2020 Census, the new tract IDs would be 10101 and 10102. And, if the new tract “10101” was split into two new tracts in the 2030 Census, the new tract IDs would be 10111 and 10112 or 10103 and 10104.

We used this renumbering system to figure out which 2020 census tracts were created by splitting 2010 census tracts. This helped us to identify 81.6% of census tract changes. 

Visually Comparing

We then visually examined the 2020 census tracts that have completely new census tract IDs and compared them with the 2010 census tracts in ArcGIS, so that we could identify which new 2020 census tracts were created by combining the 2010 census tracts.

Some of the new census tracts completely merged two or more 2010 census tracts, resulting in the combined census tracts we discussed earlier in this blog. Others only merged portions of neighboring census tracts, resulting in what we defined as new census tracts. 

Comparing Boundary Changes Using Geoprocessing Tools

The remaining 2020 census tracts had the same census tract ID as that of 2010. They were compared with 2010 tracts based on their shapes and boundaries to identify 2020 tracts that have expanded or shrunk in size but still kept their previous census tract IDs. 

Expanded census tracts are the ones that have a larger geometric shape and geographic size than their 2010 counterparts. Shrank census tracts have a smaller shape and size than their 2010 counterparts. 

This process ensures that D3 data users are always comparing the data in the same geography through the years.

Want to know how census tract changes have impacted Detroit and your community? Check out our blog about census tract changes in Michigan and Detroit.

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