Here's how it works:
Step one: Get an eye in the sky. High-quality overhead imagery can give a pretty accurate sense of how many people are in a given spot below. For the 2009 inauguration, dueling estimators used everything from aerostats to satellites to snap shots of the masses.
Step two: Take a sample. Focus in on one small part of the crowd, and get a sense of its density. University of Illinois crowd-guru Clark McPhail figures a person can comfortably stand in five square feet. In tightly-packed situations, each person can squeeze into two-and-a-half square feet. Much more than that, and it’s The Who at Cincinnati, 1979.
Step three: Measure the space. Get a sense of the square footage where the gathering is taking place. Tahrir Square is about 490,000 square feet, according to the private intelligence firm STRATFOR. At two-and-a-half-square feet per person — “comparable to the crowd density of a packed subway car” — even 200,000 activists would be pushing it. That figure sounds about right. Maybe there’s a million people protesting in Cairo. But no way could they fit into that single spot.
Mr. Shachtman also observes that crowd estimating remains controversial, regardless of the science behind it. The main reason? Event sponsors don't like the totals, particularly if the number of participants is below projected turnout. "Minister"Louis Farrakhan threatened to sue the National Park Service when the agency reported that only 400,000 people showed up for his Million Man March. After that, the Park Service stopped offering public estimates for crowds gathered on federal property.
Our guess? Some media totals will be wildly inaccurate, and (to some degree, agenda-driven). Iran's Press TV has claimed that two million people packed the square for today's demonstrations--a physical impossibility--and more than eight million protesters were in the streets across Egypt. The Associated Press puts the number in Tahrir Square at 250,000 (must have read Shachtman's article), while The New York Times pegs the crowd at "hundreds of thousands."
Unfortunately, one of the best crowd-estimation tools isn't available for today's protests in Egypt. The USAF Thunderbirds have been doing their own crowd estimates for years; what the pilots observe during their aerial demonstrations (in terms of people on the ground) is extremely close to tallies from other sources, such as the number of tickets sold, or the number of people counted entering the air show venue.
But, we're guessing red, white and blue F-16s criss-crossing the Egyptian skies would send the wrong signal. So, we're back to media estimates of crowd size, and take those with a large grain of salt.
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