There were countless heroes who helped rescue Hurricane Harvey victims last month. Most of them were in Texas, where the giant storm killed 71 people and caused billions in damage. One was a software developer visiting New York City whose mapping skills provided invaluable real-time information to Coast Guard rescuers.

Greg Sadetsky, a 35-year-old freelance software engineer, was watching from afar when the storm roared in from the Gulf of Mexico. He noticed that people around the world were working together via various online communities–Reddit, Twitter, Facebook, and Slack–to offer assistance. As someone with experience developing mapping software, he decided to build a new map he thought might be helpful to rescuers trying to navigate the ravaged area, and quickly posted it into a Slack group where volunteers were coordinating their efforts.

At first, his project got lost in the mix, but within a few hours, he tells Fast Company, he got an email from Nathan, a lieutenant in the U.S. Coast Guard. Nathan applauded Sadetsky’s map, but noted that it was missing some things that might be useful. Would Sadetsky be willing to help out the rescue efforts by adding a bunch of new functionality that could assist with the dispatch of helicopter crews?

“I said, Of course,” Sadetsky recalls, “and from that point, I was getting [frequent] feature requests from Nathan and his Coast Guard colleagues, and I was implementing those changes on the map . . . If there was something they deemed successful on the map, I added it, and we did that for about a week.”

Sadetsky began to add layers to his map–things like where calls for rescues were coming from, where helicopters were airborne, which missions had been completed, and where helicopters could land. There was also a layer showing no-fly zones–places like the area around a chemical plant, where a lack of power had eventually caused a big explosion.

The map was available on a mobile app and on the web. It allowed the rescuers to toggle between layers in order to see where they were needed next, or which of their helicopters had finished a mission and could move on to the next one. Nathan and his team kept sending Sadetsky new requests, or new information to add to the map, and updated it as fast as he could, often once a minute. By the time the storm had moved on, the map had around 40 different features.

“During that time, it took over my full days, and I was available to them 24/7,” Sadetsky says. “They had my cell phone, and I installed the Zello walkie-talkie app. I didn’t go out of the [house much], but if I did, I had my laptop, so I was available to answer any of their requests.”