Last Saturday, 440,000 people marched on Washington in support of women’s rights and against the Trump presidency.

The new estimate comes from Curt Westergard, the president of Digital Design and Imaging Service, whose company both shoots and analyzes aerial photos and has counted other large gatherings on the Mall, such as Jon Stewart and Stephen Colbert’s 2010 “Rally to Restore Sanity and/or Fear.”

Westergard’s team collected images by attaching high-resolution cameras to tethered balloons 525 feet above the National Mall. The cameras snapped photos of the entire area repeatedly from 12:36 pm until about 4 pm Eastern on Saturday. They then spent five days analyzing the images to calculate peak crowd density (at 2:46 pm Eastern) and average number of people present during the event.

Westergard told Vox they were unable to get their cameras as high as they would have liked due to cloud coverage, which means some of the photos had to be shot at a slanted angle. Another gap in the data: They didn’t get aerial photos of all the protesters gathered near the Ellipse or on 15th Street, where Westergard said there may have been 50,000 more people present.

An event that anticipated 200,000 people more than doubled in size

The Women’s March began with a viral post on Facebook calling a diverse group of women and men to demonstrate the day after Trump’s inauguration. Organizers requested a permit for 200,000 people to march, but in the days leading up to the event, they said registration skyrocketed.

As you can see in the chart below, the Women’s March on Washington joins the ranks of some of the largest civic protests to have gathered in the nation’s capital.

And Saturday’s protest wasn’t just a march on Washington. Every state in America hosted a Women’s March, as you can see in the map below.

According to Erica Chenoweth, an expert of civil resistance movements in the US, Saturday marked the largest demonstration in US history, with at least 3.3 million people in more than 500 cities protesting.

Sister marches ranged from 10,000 people in Helena, Montana, to more than 500,000 people marching in large metro areas like New York city and Los Angeles.

What’s more, it wasn’t just US cities protesting on Saturday. Another 100 international Women’s Marches joined in solidarity. Chenoweth, who has been collecting crowd data for all of the marches in the US and abroad along with political scientist Jeremy Pressman, estimates that at least 267,000 people attended these marches outside the US.

Counting crowds is intensely political

The process of crowd counting can be intensely political, which means the infrastructure to count crowds quickly and accurately simply doesn’t exist, even though the technology does.

To get a better understanding of why counting crowds on the National Mall is particularly fraught, I spoke with Farouk El-Baz, the academic responsible for settling the crowd dispute between the Million Man march organizers and the National Park Service (NPS) in 1995.

March organizers had estimated that nearly 1.5 million to 2 million people attended the 1995 march, whereas the NPS estimated only 400,000 had. Farouk found the number was closer to 837,000 people after analyzing footage from the event.

Farouk did more than just calculate an accurate number for people in attendance — he made a series of recommendations for what was needed to help the NPS improve its methods of crowd counting. He recommended the NPS have access to aircraft with mounted cameras that could take photographs at hourly intervals.

When the NPS brought his proposal before Congress with a hefty price tag of $20 million, Congress denied the proposal. And in a 1997 appropriations bill, NPS was forbidden to spend federal funds on counting crowds. But Farouk said counting crowds like those at the Women’s March is doable — and because the technology has only improved since 1995, he thinks it should be done.

“There will always be a discussion of the results because all sides will be discussing to say no that can’t be, [i.e.] I saw a million people myself,” said Farouk. “The only way to stop the controversy is to get the information systematically and at a specific time.”

Watch: Millions joined rallies around the globe