Since a massive 7.8 magnitude earthquake struck Nepal on April 25, over 7,000 people have died, and many more have been injured or left stranded in rural areas. Aid groups like the Red Cross and Doctors Without Borders have deployed teams to help those left behind in the districts of Dhading, Gorkha, Rasuwa and Sindhupalchowk. But there are plenty of people who are contributing from thousands of miles away—on their lunch break, after work, or on the weekend. They’re part of an online community of volunteers from all over the world who are mapping Nepal from their laptops, creating data that’s critical to on-the-ground relief.

More than 4,000 mappers—part of OpenStreetMap (OSM), the biggest crowd-sourced mapping project on the Internet—joined in the effort. Within 48 hours, they had mapped out 13,199 new miles of road and 110,681 buildings, quadrupling the road mileage covered and adding 30 percent more buildings. Largely building off of satellite images and GPS data, they detailed a huge swath of the region, providing critical information about road networks, hiking trails, relief camps, footpaths, and river crossings to governments and aid organizations. “The maps will be used in all kinds of ways to deliver aid," says Tyler Radford, interim director of the humanitarian OSM team, "whether it’s healthcare, food, or shelter."

Those are amazing results for a humanitarian team run almost entirely by volunteers, and almost entirely remotely. Capitalizing on any time its volunteers can offer—from 20 minutes to a whole work day—OSM has organized efforts in crises like the 2010 Haiti earthquake and the Ebola outbreak. Many of its contributors have no expertise in mapping, but the system is simple enough that a quick online tutorial can get them started, and their work is reviewed by more experienced users.

Several hours after the earthquake hit Nepal, OSM had already activated its network, which it deployed strategically to map the areas with the least coverage that had also been hit the hardest. Seismological data and reports from the ground—from a local mapping group, the news, as well as social media like Twitter—helped its coordinators identify damaged areas, so volunteers could focus first on adding escape and delivery routes there. Requests also came in from aid organizations, the Nepalese Army, and the government for maps that could help them identify and access victims.

The first need: maps of roads and buildings. The local group, Kathmandu Living Labs, had already mapped out a lot of the city (you can see the dense, filled-in city areas in white in the maps above). But volunteers had a lot of work to do filling in the districts just outside the city and rural areas. They also identified potential helicopter landing spots, mapped unrecorded footpaths into remote villages devastated by the earthquake, and used post-quake satellite images to identify camps that people set up after their homes collapsed.

As basic map information is gathered by the volunteers, OSM makes it immediately available to the public—so that people can mesh it with other datasets to create custom maps, like one that identified the location of Nepalese healthcare facilities. The foundation built by OSM's volunteers will not only make it easier to get help to victims of this disaster, but hopefully those of future disasters as well.