On Friday, hundreds of thousands of people made their way to downtown Washington, D.C., to celebrate the inauguration of Donald Trump. The next day, thousands more will make the same journey for the Women’s March on Washington.

In the days following both events, the press will report their total attendance. But how will journalists know how many people attended?

Crowd counters rely on a few methods. For the inauguration, they will likely use a single aerial photo of the crowd, captured by a helicopter or satellite. Experts will augment their knowledge of how many people can fit into a space with some “head-counting”—literally, going person by person.

This is how the estimate for the 2009 inauguration was reached. Stephen Doig, a professor at Arizona State University, consulted a satellite image released by the company DigitalGlobe and concluded that about 1.1 million people had watched the ceremonies from the National Mall. But the D.C. government, working off the same satellite image and the reports of federal and municipals employees, arrived at a much larger figure: 1.8 million.

This large gap exists in part because the best, most reliable methods of crowd counting cannot be used for the inauguration. The most reliable methods rely on a static camera that observes the entire event, throughout its full duration. Because airspace is closed during the inauguration, that isn’t possible on Friday.

But those techniques can be used on Saturday. Curt Westergard is an expert on them and will use them for the Women’s March. He is the president of Digital Design and Imaging Service based in Falls Church, Virginia, and he stressed that his company’s methods were “at the very top of the accuracy and ethical side.”

He and his colleagues will be armed with the chief tool of their trade: a weather balloon.

Well, technically, a “tethered aerostat.” Tethered because it is anchored to the ground, and aerostat because it will hold a static altitude in the air. A nine-lens camera is attached to its base, so it can capture the full 360-degree view of the proceedings. It will observe the entire Women’s March.

A tethered aerostat is superior to a satellite for several reasons, but one of the most important is that it always takes photos below the cloud level.

Westergard and his team have crowd-counted many marches in the Mall over the last 12 years, including Sarah Palin’s “Restoring Honor” protest and Jon Stewart and Stephen Colbert’s “Rally to Restore Sanity and/or Fear” in 2010.  

Their technique involves more than just the weather balloon. While the weather balloon records the events from above, Westergard and his team will bike or walk around the protest site. They’ll take note of how many people are taking cover under structures, like the massive elm trees on the Mall. Sometimes they’ll even lower the aerostat so that it can capture crowds in the shade. “At 400 feet, we’re looking under the trees. At 800 feet, you’re looking at the top of them,” he told me.

A stylized version of a grid used by Westergard’s team
(Digital Design and Imaging Service)

Once the data is collected, they return to their headquarters. Three days of work commences. First, they will measure the density of different parts of the crowd. They do this by counting heads in a specific area. “We sit there literally, head by head, going tick-tick-tick-tick-tick” with the images, he told me. “It’s painful, it’s long, but it’s far more accurate than these algorithms.”

Sometimes they outsource this task to Amazon’s Mechanical Turk service to increase their own accuracy: They ask a dozen strangers to count heads in a certain picture without telling them where the picture was taken.

Once they have this density map, they overlay it on a map of the topography. “If you have people surrounding the Washington Monument—which is on a moderately steep hill—and you look out at a crowd, you’re going to see more people because they’re tilted toward you,” he said. The computer model will correct for those kinds of inaccuracies.

Ultimately, his team will produce a range of how many people were at the event. (He cautioned that I should never call this an “estimate.”) This will essentially be a maximally accurate statement of the crowd’s carrying capacity: a statement that no fewer than 300,000 people could have been at a certain event, but no more than 600,000 either.

“It proves or disproves order of magnitude, which is useful,” Westergard said. “A million is different than a hundred thousand.”

This range is heavily based on what the likely density of people will be in the rally area.

“At her rally, Sarah Palin said, ‘There’s a million people here.’ A million people, at one person per 25 square-feet—that’s the area of you reaching your hands out and touching someone at your fingertips—that million people would extend across the Potomac River into Virginia,” Westergard said.

Even a crowd at one person per 9 square-feet—“that’s you standing on a Washington Post newspaper right below your feet, and your elbows touching your neighbors”—would extend into the Potomac River if it exceeded 1 million people, he said. These basic reality checks are how he hopes his ranges can be useful.

He cautioned, too, that crowd density does not always work how people suspect it does. Most people assume that crowds are dense near the stage and then peter out. In fact, he said, crowds mostly gather near Jumbotrons.

Crowd-counting has been controversial in the past. In 1995, the National Park Service reported that only 400,000 people attended the Million Man March in Washington, D.C. Organizers balked. When remote-sensing analysts at Boston University reexamined the evidence, they found that 837,000 people attended, plus or minus 20 percent. The National Park Service no longer issues public estimates of crowd size.

Westergard’s team counts more than crowds. They have also deployed the aerostat to measure the physical size of oil spills. They use it to show people what the view from a skyscraper will look like before the skyscraper is built.

Before he was in the aerostat business, Westergard was a landscape architect. “I’ve always been interested in quantifying organic things, like how many geese are flying by, or how many square feet of wetlands there might be,” he said.