The real estate company Zillow has assembled county-level cadastral and tax assessor data to an impressive, U.S.-wide database (ZTRAX), holding over 400 million assessment and transaction records, including detailed parcel-level information on building age, size, usage, value, and physical characteristics (https://www.zillow.com/research/ztrax/). Zillow has made these data available for researchers through data share agreements. Since 2017, we work with this dataset and have developed methods to ingest ZTRAX into efficient relational databases and to extract geospatial datasets, which we can then aggregate into US-wide, high-resolution gridded data for any attribute of interest. Based on the “year built” reported in ZTRAX for a large majority of building records we are able to generate time series of such gridded surfaces representing the built environment at different point in times.
These gridded surfaces enable the long-term mapping of U.S. urbanization at unprecedented coverage and detail:
All gridded datasets are publicly available at https://dataverse.harvard.edu/dataverse/hisdacus.
Methods & results
From the HISDAC-US gridded datasets we generated multivariate time series for over 300 metropolitan areas, describing the size, shape, and structural characteristics of each urban area, in 5-year intervals from 1910 to 2010 (get the data here: https://doi.org/10.6084/m9.figshare.13303091.v1). In our publication "A century of decoupling size and structure of urban spaces in the United States" (https://doi.org/10.1038/s43247-020-00082-7) we used multivariate statistical methods such as t-distributed neighbor embedding and cluster analysis to characterize the trajectories of urban areas over time, and to identify similarities between urban areas. We observed an increasing disentanglement of size- and structure-related characteristics over time, and identified a recently emerging cluster of urban areas characterized by high levels of contiguity, as a result of growing and merging processes, connecting built-up areas that initially were spatially disconnected. Such an disentanglement indicates that while cities show similar changes in size, their inner structure (urban form) might evolve in different ways. This can be significant for our urban life because changing structures would require different policies for transportation planning or public and health service supply.
These datasets do not only open up new pathways for analytical, quantitative assessments of urban growth and long-term land development, but also allow for effective visualization of land development and urban growth, at unprecedented spatial detail and spatio-temporal coverage. We generated aspatial and spatial data animations based on our city-level time series and the time series of gridded surfaces. The animation below illustrates the dynamics of city sizes (measured by the total built-up area) during the 20th century:
See here some HISDAC-US movies:
See more in our HISDAC-US YouTube channel (https://www.youtube.com/channel/UCYChFz-8stVzTVyqazXbqOQ).
US-wide land development trends over the long term
Besides these insights into urban spatial development, the HISDAC-US data also provides vast opportunities to study land development across the United States as a whole, including sparsely settled rural areas. The fine spatial detail allows us to divide land development into components such as expansion, infilling, and densification, and to quantitatively assess how these components have interacted during the 20th century and beyond. The regional relative growth curves (shown below) peak at different points in time, reflecting region-specific settlement patterns. Future work will examine such development patterns at local scales.
Measuring the past to predict the future
Future work will focus on the development of spatially explicit urban (and rural) growth models, informed by the spatio-temporal settlement dynamics measured through the HISDAC-US data, integrated with other spatial data sources such as road network data or historical maps (https://usc-isi-i2.github.io/linked-maps/). Moreover, these datasets and the underlying ZTRAX database enable us to assess how the U.S. built environment and the building stock interact with natural hazards (flooding, fire risk) and social vulnerability, to better understand the dynamics of socio-environmental processes over the long term. The animation below illustrates how urban change (e.g., land conversion from not built-up to built-up land) can be measured over long time periods:
Web links and references
Connor D., Clement K., Cunningham A., Gutmann M. and Leyk S. 2020: How Entrenched is the Spatial Structure of Neighborhood Inequality? Evidence from the Integration of Census and Housing data for Denver from 1940 to 2016. Annals of the American Association of Geographers 110(4): 1022-1039, https://doi.org/10.1080/24694452.2019.1667218
Leyk, S., Uhl, J. H., Connor, D. S., Braswell, A. E., Mietkiewicz, N., Balch, J. K., & Gutmann, M. (2020). Two centuries of settlement and urban development in the United States. Science Advances, 6(23), eaba2937, https://doi.org/10.1126/sciadv.aba2937
Leyk, S., & Uhl, J. H. (2018). HISDAC-US, historical settlement data compilation for the conterminous United States over 200 years. Scientific Data, 5, 180175. https://doi.org/10.1038/sdata.2018.175
Mietkiewicz N., Balch J., Schoennagel T., Leyk S., St. Denis L.A., Bradley B.A. 2020: In the Line of Fire: Consequences of Human-Ignited Wildfires to Homes in the U.S. (1992-2015), Fire 3(3), 50, https://doi.org/10.3390/fire3030050
Uhl, J. H., Connor, D. S., Leyk, S., & Braswell, A. E., (2021). A century of decoupling size and structure of urban spaces in the United States. Communications Earth and Environment, 2, 20. https://doi.org/10.1038/s43247-020-00082-7
Uhl, J. H., Leyk, S., McShane, C. M., Braswell, A. E., Connor, D. S., & Balk, D. (2020). Fine-grained, spatio-temporal datasets measuring 200 years of land development in the United States. Earth System Science Data, 13, 119–153. https://doi.org/10.5194/essd-13-119-2021
Data visualization and animation:
HISDAC-US Youtube channel: https://www.youtube.com/channel/UCYChFz-8stVzTVyqazXbqOQ
Uhl, Johannes H.: Animating 200 years of urban spatial development in 35 U.S. cities. Uhl, Johannes H.. 2020. https://doi.org/10.5446/48115
Bar chart race: built-up area per MSA, 1910-2010: https://app.flourish.studio/visualisation/2765392
Bar chart race: built-up intensity per MSA, 1910-2010: https://app.flourish.studio/visualisation/2765807
Bar chart race: number of built-up patches per MSA, 1910-2010: https://app.flourish.studio/visualisation/2765858
Data access & source data:
HISDAC-US data repository in Harvard Dataverse: https://dataverse.harvard.edu/dataverse/hisdacus
Urban spatial metrics for U.S. Metropolitan statistical areas (1910-2010): https://doi.org/10.6084/m9.figshare.13303091.v1
Zillow's Assessor and Real Estate Database (ZTRAX): https://www.zillow.com/research/ztrax/
Publicity & News:
Business Insider (2020): “A stunning new video shows how cities exploded onto the US map over the last 200 years” (https://www.businessinsider.com.au/us-urban-development-density-last-two-centuries-2020-6)
Financial Times (2020): "Mapping how railroads built America" (https://www.ft.com/video/28ef4d98-ac46-4b68-84da-3ef5cc70e730)
Colorado Arts and Sciences Magazine (2018): "CU Boulder geographers studying plot-level land-use changes over 200 years (https://www.colorado.edu/asmagazine/2018/09/04/cu-boulder-geographers-studying-plot-level-land-use-changes-over-200-years)
Colorado Arts and Sciences Magazine (2020): "New CU Boulder research provides unprecedented opportunity to study history and evolution of human land-use and development in the United States" (https://www.colorado.edu/asmagazine/2020/06/02/mapping-200-years-american-development)
ASU Now: Access, Excellence, Impact (2020): "New ASU research empowers efforts to plan smarter, more resilient cities (https://asunow.asu.edu/20200603-new-asu-research-empowers-efforts-plan-smarter-more-resilient-cities)