Thursday, August 20, 2015

The Biggest U.S. Cities by Decade

Since the early years of the United States there has been a gradual westward shift in the mean center of population. In 1790 the most populated cities in the U.S. were all on the East Coast. In the 2010 census seven of the ten largest cities were located in the Sun Belt region of the south and west.

A few weeks ago I saw a really interesting animated GIF, posted to Reddit, which mapped the top ten cities by population by decade in the United States. You can view the map, by Reddit user Eudaimonics, here. Being an animated GIF means that you can't interact with the map. I really wanted to play with the map so I decided to create my own interactive map of the same data.

Shifting Cites shows the top ten most populated U.S. cities for every decade since 1790. The map also shows the mean center of population in the USA for each decade. The data for the map comes from each U.S.census as listed on these two Wikipedia articles; Largest cities in the United States by population by decade & Mean center of the U.S. population.

To create the map I made extensive use of the Leaflet mapping library's Layer Groups feature to group cities by decade. The map then simply uses an HTML5 range slider control to load the markers onto the map by the relevant decade.

When you select a decade from the slider control the blue markers show the location of the top ten most populated cities and the red marker shows the mean center of the population. The left hand side panel also updates to show a numbered list of the top ten cities for the selected decade. The small window in the top right-hand corner of the map also updates to provide more general information on the patterns of population movement being shown on the map.

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