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Maps

Mapping the 'Time Boundaries' of a City

Maps don't typically convey time very well. They're static snapshots of a moment in history. They tell you what exists, not when people go there, or how the value of a place might be tied to time – whether it's a nightlife district or a public park most popular with early-morning joggers.

We've come across a handful of animated maps that do a good job combining time and space, frequently using either transit data or geo-tagged social-media hits. Now a new project, called Geographies of Time, is trying to do something similar with a more typical two-dimensional map. The effort is part of a broader EU-funded project called UrbanSensing that's building platforms to detect patterns in how people use urban spaces.

With Geographies of Time, the researchers wanted to erase how we typically think of boundaries within cities – between neighborhoods, for instance – and replace them with new ones dictated by time. Which parts of a city come alive between midnight and 3 a.m.? How about at lunch time? And what might those patterns tell us about how individual places – and whole cities – are experienced differently over the course of a day?

Giorgia Lupi, the Ph.D. researcher at Milan Politecnico behind the project, began with Milan. Using tens of thousands of geo-tagged tweets, she and colleagues divided the map of the city into a fine-grained grid. The tweets were then divided into eight three-hour time intervals (from midnight to 3 a.m., 3 a.m. to 6 a.m., etc.). And the boxes in the grid were digitally colored based on the time window when Twitter was locally most active. Here, for instance, is a representation of a weekday in Milan, with the key at right:

And a weekend:

For comparison, Lupi and her information design agency Accurat prepared another set of maps from New York City for us, using one month of geolocated tweets from the city in July of this year (covering about 3 million tweets total). On this map, the individual squares are miniscule, about 250 by 350 feet. But to reduce anomalies in the map, squares are colored based on the tweet activity in the eight squares surrounding them as well. As with Milan, there are eight three-hour units of time over the course of the day.

This is New York on a weekday:

And a weekend:

Zooming in, you can see heavy mid-day activity around lunchtime in the Financial District on week days:

On a weekend, there are a lot of night-time revelers near Times Square:

And the area around Yankees Stadium lit up around 6-9 p.m. on weekdays, adjacent to Macombs Dam Park:

The result looks a lot like Tetris. But it's certainly effective in re-scrambling how we think about the divisions within cities when we look at a map.

Emily Badger is a former staff writer at The Atlantic Cities based in Washington, D.C. She now writes for The Washington Post. All posts »

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