Kyle Pennell
Nov 9, 2021

Mapping 30 Years of Census Data with Dot Density in OmniSci

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I remember the first time I saw a dot density map. It was back in 2010 when the University of Virginia released the racial dot map with that years' decennial census. 

Transfixed with how the map represented population distribution, I observed the effects of segregation, immigration, and gentrification like never before. When a dot symbolizes a person (or 100 people), there's just something surreal about recognizing how and where we've clustered across the continent. 

Fast forward to today, and the US 2020 Decennial Census results are slowly starting to come in. The redistricting results landed in August 2021, and the newest set of American Community Survey (ACS) data will come out in late November 2021. 

This data has enormous public policy implications like congressional redistricting, representation, gerrymandering, and a general understanding of US population distribution and how demographics change locally and nationally.

Dot density maps are an intuitive method for demonstrating how humans cluster together or disperse across an area, which is particularly valuable when visualizing demographic data or voting results. 

While they've been used for some time, dot density maps have recently gained more clout with the 2010 Racial Dot Map, the New York Times' Mapping Segregation, or even this cheeky XCKD map (with 250k people per dot) of the 2020 election results. 

The challenge with most dot density maps is they're often slow (hundreds of millions of points is a lot!). They aren't very interactive beyond zooming/panning and don't allow us to understand changes over time. We wanted to fix this, and thanks to OmniSci, we can.


Houston 1990 - 2020


We took demographic dot density maps one step further by creating a set of Dot Density Dashboards. These dashboards have data from the 1990, 2000, 2010, and 2020 censuses, representing over a billion points at the one person to one dot resolution. 

With these three decades of data and intuitive maps and charts, we end up with a set of dashboards that allow us to explore and interrogate 30 years of US population changes at the very granular census block group level.

To best show you all the results and how we made it possible, we've prepared a three-part blog series, a collection of videos, and published a public demo so you can explore your neighborhood or hometown.

This three-part blog series will show you the following: 

  1. The first part (the remainder of this post) will explore some of the surprising knowledge gained from the analysis and visualizations.
  2. The second part will discuss how we acquired the data for each census and the Python notebooks used to turn the census block group polygons into individual points.
  3. The final part will explore how we created the charts, maps, and dashboards in OmniSci Immerse. 

Let's jump into the results!

Knowledge Gained

This part of the series will focus on interesting patterns and themes we discovered using the dot density dashboard. 

Population Losses and Gains

It's fun to use the map lasso tool to see overall population trends over the last three decades. You can see that the Southwest and Texas have nearly doubled in population in that time.


Population growth in the southwest 1990 - 2020


Whereas the midwest's population increases have been much lower.


Population growth in the midwest 1990 - 2020


Metro areas like Phoenix have seen considerable increases in population.


Population growth in the Phoenix metropolitan area 1990 - 2020


While areas like downtown Detroit have seen declines.


Population decline in the Detroit metropolitan area 1990 - 2020


New Neighborhoods Popping Up

Using the 'Approximate population by Year' New Combo Chart allows brushing through the different years of census data. Going from 1990 to 2020 shows you how neighborhoods and cities have changed over time. 

It's fascinating to see areas of cities that used to have no one living in them suddenly become new neighborhoods. The graphic below shows how the Mission Bay and SOMA areas of San Francisco added housing over the decades.


Neighborhood changes in San Francisco 1990 - 2020


A similar thing happened in the DUMBO area of Brooklyn, NY.


Neighborhood changes in Brooklyn, NY 1990 - 2020


Chicago has multiple blocks that were empty in 1990, fill in with residents by 2020.


Neighborhood changes in Chicago, IL 1990 - 2020
Neighborhood changes in Chicago, IL 1990 - 2020


Demographic Shifts

When zoomed out, we can see how much the overall white population percentage has declined while the Asian, Hispanic, and two or more races have increased. 


US Census data 1990 - 2020


It gets even more interesting to see how these demographic shifts play out on a regional or city level. The above trend, for example, is much more noticeable in a place like California.


California Census Data 1990 - 2020


And Arizona…


Arizona Census Data 1990 - 2020


Gentrification and Displacement

The previous three decades have been an era of rapid urbanization across much of the US. One unfortunate part of this has been the displacement of prior, often marginalized community residents from many urban cores. 

The dashboard shows just how drastic this has been in some neighborhoods. West Oakland, for example, went from 78% black to 32% black (percent of the total population) over the previous three decades. 


Oakland, CA Census Data 1990 - 2020


The Ocean View and Ingleside areas of San Francisco went from nearly half black to single-digit percentages in that period.


Ocean View and Ingleside Neighborhoods Census Data 1990 - 2020


Harlem, New York, saw a similar pattern.


Harlem, NY Census Data 1990 - 2020


Meanwhile, places like Atlanta and Houston saw their black populations hold steady or increase. 


Atlanta, GA Census Data 1990 - 2020


Houston, TX Census Data 1990 - 2020


More Questions Than Answers

Having all of this data in one place will further the types of conversations we can have about population and demographic shifts. As with any proper data exploration project, this one provides at least as many questions as answers. Here are some other things worth exploring, perhaps in future pieces:

  • How have the origin countries of different Hispanic immigrants changed over time? It would be interesting to see the patterns that come up after visualizing census variables like 'Total!!Hispanic or Latino!!South American!! Venezuelan' or 'Total!!Hispanic or Latino!!Central American!!Salvadoran'.
  • How has the median or per capita income changed over time? We could style the point size according to these variables and probably see some fascinating results.
  • Similarly, how would variables like age, employment status, owner vs. renter play out in this sort of visualization? 
  • The decennial census features hundreds of variables, and most of them would be interesting to visualize in a dot density format.

We'd love to hear about your findings once you’ve used the dashboard, as well as any feedback you may have. Please share those thoughts and experiences with us on  LinkedInTwitter, or our Community Forums!

Solutions Engineer at OmniSci