# Estimating big crowds accurately with weather balloons, bicycles, and counting

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…

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…

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.

See earlier posts (such as here and here) about counting crowds.

It is also interesting that this more accurate method is explained by the leader of a private firm: “Curt Westergard…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 is working for those who want to hire him, something that could be worthwhile for the article to explore. Are they impartial observers who are doing this work for science? In other words, crowd counting could be influenced by who exactly is doing the counting. Parties who often make the counts – police, local officials, the media – have vested interests. For example, take the case of the rally for the Cubs World Series victory.

Of course, as it noted in this article, the numbers themselves are often politicized. What will be the official count accepted by posterity for a Trump inauguration that likely stirs up emotions for everyone?