Which population matters to planners, businesses and service providers? The number of inhabitants only captures part of the picture. New maps capture commuter data to show how cities grow or shrink during the day as people commute in or out for work. For example, Manhattan's population of about 1.5 million doubles to around 3 million every day.
As Emily Badger explains it:
If Manhattan ever needs to evacuate by day during a disaster, the city has to figure out what to do with all 3 million of those people. The city's transportation planners are responsible for every one of them, whether they live in New York or not. And anyone who does business in a service industry on the island – from lunch counters to dry cleaners to department stores – cares a lot more about how many people pass through during the day than who passes out in Manhattan at night. [...] This geography of how populations move on a daily basis should also tell us something about the importance of regional transportation infrastructure. If your city swells in size every day by 50,000 people or more, do you want all of them coming by car?