NASA would like you to take a picture of a tree, please. The space agency’s ICESat-2 satellite estimates the height of trees from space, and NASA has created a new tool for citizen scientists that can help check those measurements from the ground. All it takes is a smartphone, the app, an optional tape measure, and a tree. So to help, the Verge Science video team went on a mission to measure some massive trees in California as accurately as they can.

Launched in September 2018, the ICESat-2 satellite carries an instrument called ATLAS that shoots 60,000 pulses of light at the Earth’s surface every second it orbits the planet. “It’s basically a laser in space,” says Tom Neumann, the project scientist for ICESat-2 at NASA Goddard Space Flight Center. By measuring the satellite’s position, the angle, and how long it takes for those laser beams to bounce back from the surface, scientists can measure the elevation of sea ice, land ice, the ocean, inland water, and trees. Knowing how tall trees are can help researchers estimate the health of the world’s forests and the amount of carbon dioxide they can soak up.

But Neumann says that a big open question is how good those measurements from space actually are. That’s where the citizen scientist comes in — to help verify them. Some are more challenging than others. “You can’t really ask a bunch of school kids in Pennsylvania to go to Antarctica to measure the ice sheet height for you for a calibration,” he says. But you can ask them to take their smartphones outside, which is exactly what NASA is doing with its GLOBE Observer app. “You’ve got all sorts of great terrain and features right in your backyard that you could go out and do these measurements that would be useful for us,” Neumann says.

The NASA GLOBE Observer app is pretty user-friendly. After you download it, you can choose from different tools that record cloud observations, mosquito habitats, and the landscape around you. There’s also a new tool for measuring trees, called GLOBE Trees. When you first open it, a tutorial walks you through how to calibrate the app and take the measurements that let it triangulate tree heights. The tutorial includes helpful tips for things like “Selecting a tree” — apparently, bent and broken ones don’t get measured.

Once you’ve selected your non-broken tree and staked out a spot about 25 to 75 feet away, you hold the phone right in front of your face and angle it to measure the base and then the tree’s top. Then you take a picture, count your steps to the tree, log your position at its base, and the app spits out the tree’s height. Verge Science’s Cory Zapatka and Will Poor took the app for a test drive in Central Park and across the country in Northern California, home to some of the tallest trees in the world. They checked the app’s math by trying some of the measurements manually with a tape measure and an angle-measuring tool called a clinometer. To see how that turned out, check out the video.

Since the official launch at the end of March, GLOBE Trees has received about 700 measurements from around 20 different countries, according to senior NASA Earth Science outreach specialist Brian Campbell, the Trees science lead. And the researchers would love to get even more. The measurements are useful data for the ICESat-2 team, Neumann says. So when it comes to people using the app, he says, “The more, the merrier.”

Update May 7, 2019 10:00 AM ET: This story was originally published at 10:44AM ET on April 15, 2019. It has been updated to include details about Verge Science’s new video.