Corcoran et. al

Looking back over the history of the crowdsourced digital street map, a familiar pattern emerges.

OpenStreetMap is a marvel of modern crowdsourcing. Since its creation in 2004, DIY cartographers – typically armed with GPS devices or satellite photography – have been slowly mapping the world's road networks and landmarks to create a free alternative to proprietary geographic data that can then support tools like trip planners. The process, which began in the U.K., is painstaking and piecemeal, and nearly a decade into it, more than a million people have contributed a sliver of road here or a surveyed cul-de-sac there.

Academics refer to this kind of collaborative mapmaking as "volunteered geographic information," and OpenStreetMap is one of the most successful examples of it out there. Research into the system suggests that these amateur maps are impressively accurate in communities dense with contributors (like Germany: Germans love OpenStreetMap). But until now, it's been much easier to assess how good these maps are than to ask how they got that way.

Now, researchers are getting much better at processing OpenStreetMap's data to access its history. The above historic timelapse comes from a study, published in the journal Spatial Statistics, that retraced the growth of OpenStreetMap networks in three areas of Ireland to understand how the networks are built. This is the full visual history of the expansion of OpenStreetMap in Maynooth, Ireland, from January of 2007 through January of 2011:

From "Analysing the growth of OpenStreetMap networks" by Corcoran, Mooney and Bertolotto in Spatial Statistics. The figures correspond to the dates 01-11-07, 01-02-09, 01-03-09, 01-07-09, 01-09-09 and 01-10-11.

That evolution looks an awful lot like how physical street networks themselves evolve over time. And, in fact, the Irish researchers behind this project found that OpenStreetMap expands precisely through two processes – "exploration" at the outer edges of the known network, and concurrent "densification" of its core – that have been used to describe how road networks are built.

"In a sense, there’s a parallel between how cities grow in reality and how they are mapped," says Padraig Corcoran, one of the study's authors. This is probably not a coincidence. "It kind of suggests there might be some underlying process which governs the growth of networks in general, irrespective of what they represent," Corcoran adds.

And this could be true of social networks, street networks or networks of digital data. If you look at your own social network, for instance, your circle of friends expands outward as one friend introduces you to another, who introduces you to yet another person not already in your network. At the same time, though, your social network may also be growing "denser," as you meet people already connected to other friends that you know (i.e., the boyfriend of your roommate, who happens to also be the coworker of your college friend).

On OpenStreetMap, this is what that process looks like (again, in Maynooth), where the densification is represented in green and the exploration in red:

Obviously, a new area must be explored before anyone can build out that particular neighborhood's spiderweb-like side streets. But these two processes occur simultaneously in different parts of the network. Given that OpenStreetMap appears to so closely mirror the growth of real-world infrastructure, the tool may offer the opportunity to understand the expansion of networks in a way that no one was trying to do when these streets were paved in the first place.

Researchers have gone back to look at earlier maps of development. But that process typically involves digging out old paper maps, scanning and digitizing them. And good time stamps aren't always available to pinpoint a map's moment in time. But with OpenStreetMap, Corcoran says, "It opens an opportunity to study how networks are growing in real time. OpenStreetMap is currently an evolving process – it's still not complete. We don’t know when it will reach a stage of maturity."

All of the impressively intricate work it will take to get there raises another question that research has not yet answered about this crowdsourced process: Why are all of these people doing this?

"We don’t really know what motivates people," Corcoran says. When people register to contribute to OpenStreetMap, the site doesn't ask them why they want to participate.  "Some of the reasons people suggest is that people like to be part of a community, like to map their own area, they like to make contributions to society, to provide data that's of use to society," Corcoran says. Because there's no central authority assigning tasks or coordinating all those contributors, it seems all the more impressive that they somehow self-organize according to the same principles that entire cities exhibit when they expand over time.

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