How do you measure a crowd as big as the one at the Obama inauguration?

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Jan. 21 2009 6:24 PM

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How do you measure a crowd?

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A satellite image of the National Mall. Click image to expand.
A satellite image of the National Mall

Vast crowds filled the National Mall on Tuesday to watch Barack Obama take the oath of office. Official figures have not yet been released, but there's widespread speculation that yesterday's event broke the attendance record set by the 1.2 million people who supposedly showed up at Lyndon B. Johnson's inauguration. The Associated Press estimated Tuesday's assembled masses at "more than 1 million," the Washington Post projected 1.8 million, and CBS reported "between 1.8 million and 2 million." How do you measure a crowd?

Basic arithmetic. Estimates depend on three variables: the area of the available space, the proportion of the space that's occupied, and the crowd's density. While the first measurement is objective, and the second fairly easily determined with aerial photography, the third is a little trickier. It's customary to assume that in a very crowded place (like a commuter train during peak hours) people occupy 2.5 square feet, whereas in a looser gathering each person takes up more like 5 square feet.

This area-based method dates back to the late 1960s. After rowdy students gathered at Berkeley's Sproul Hall Plaza in December 1966 as part of the Free Speech Movement, police estimated a crowd of 7,000 to 10,000. Newspapers repeated the range, but readers were skeptical. Then Herbert A. Jacobs, a Berkeley professor, tried to arrive at a more exact figure using an enlarged aerial photograph of the demonstration. He divided the photograph into 1-inch squares and counted heads using a magnifying glass, eventually reaching a total estimate of 2,804. So that he wouldn't need to repeat this painstaking process, he deduced the average square footage taken up by each student—about 4 square feet at a tightly packed outdoor event—and confirmed this estimate at subsequent rallies.

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Thanks to advances in aerial digital photography and computer image-processing, it's now possible to get a fairly exact head count—without a magnifying glass. As Farouk El-Baz of Boston University explained in a 2003 Wired article, the best way to obtain an accurate image is to fly over the assembly at peak time and take a digital photograph (resolution 1 foot per pixel) from 2,000 feet or less. Using satellite images, an Arizona State University professor calculated that about 800,000 people attended the inauguration Tuesday—considerably fewer than the AP estimate (based on photographs and comparison with past events) and less than half the Washington Post number (based primarily on security agencies on the ground).

The National Park Service announced prior to the inauguration that they would, eventually, release official attendance figures—which is unusual. In 1995, there was a public disagreement between the Park Police and Louis Farrakhan over the Million Man March. The Park Police, using pictures taken from a helicopter, gauged the crowd at 400,000, whereas Farrakhan insisted more than 1 million were in attendance, and he threatened to sue. Shortly thereafter, Congress told the Park Service to stop issuing estimates.

Got a question about today's news? Ask the Explainer.

Juliet Lapidos is a former Slate associate editor.

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