During the Chinese New Year, people traditionally return to their families in villages all over China. 30 years ago, this event triggered the migration of about a million people over just a few days.

The rapid growth of the Chinese economy has changed all that. During the latest celebration in January and February 2014, some 3.6 billion people travelled across China making this the largest seasonal migration on Earth.

That raises an interesting opportunity. The Chinese web company Baidu offers a smartphone mapping app used by 200 million people that records their location over time. Today, Xianwen Wang and pals at the Dalian University of Technology in China, use this data to visualise this migration in remarkable detail for the first time.

The Baidu data consists of location records for every smartphone using the company’s mapping app between 16 January and 18 February 2014. The data is gathered hourly and any change in location noted. At the time, Baidu published a travel map showing the most popular routes in real time for the 200 million people signed up to its mapping service.

Xianwen and co have gone further by cutting and dicing the data to show the direction and size of migration to and from the various regions in China over the entire festive period.

The results show that the mass movement of people begins about a week before the Chinese New Year’s day on 31 January. This movement peaks on 26 January and then falls dramatically to a minimum on New Year’s Eve.

This is followed by a public holiday lasting until 6 February, when a second, even higher, peak in travel occurs, as people in government jobs return home to begin work. Travel activity remains high until 14 February, probably because students and migrant workers can take longer breaks.

“The migration direction is completely opposite before and after Chinese New Year’s Day,” say Xianwen and co. “Before New Year’s Eve, people return their hometown for reunite with their family. After that, they would go back to their work place.”

The data shows some interesting variation between different regions. The largest migration occurs in Guandong province on the South China Sea coast. This is China’s most populous region with about 100 million residents, of whom 30 million are migrant workers attracted by the region’s many factories.

Xianwen and co say the data shows some 2.5 million people leaving Guangdong in the days before New Year. The increase in travel activity begins earlier and last longer than in regions such as Beijing. That’s because many of the travellers are migrant workers.

By contrast the majority of travellers from Beijing and Shanghai are government employees who find it much harder to negotiate leave beyond the public holiday period.

Xianwen have used the data to produce interactive diagrams showing the direction and size of travel activity from one region to another.

That’s interesting work that exposes the details of the largest migration on Earth for the first time. Xianwen and co do not describe how this data could be used but obvious ideas are for planning the country’s infrastructure and travel-related resources for next year.

It might also be useful in case of the outbreak of disease at this time of year. The data clearly shows how people, and therefore disease, spread around the country and so might offer some indication of how best to mitigate the spread.

The Chinese researchers mention the question of privacy, however. That’s an issue that would make this kind of tracking difficult in democratic countries, or at least the public acknowledgement of it.

Ref: arxiv.org/abs/1411.0983 : Tracing the Largest Seasonal Migration on Earth