Jacquelyn Martin/Associated Press
The crowd attending the "Restoring Honor" rally, organized by Glenn Beck, is seen from the top of the Washington Monument in Washington, on Saturday, Aug. 28, 2010.
On Saturday, a large crowd converged on the National Mall in Washington, for conservative commentator Glenn Beck's "Restoring Honor" rally.
How large was it?
Event organizers secured a permit from the U.S. National Park Service for a 300,000-person crowd. Afterward, Beck said its size ranged from 300,000 to 600,000.
CBS News, which commissioned aerial photography of the event, said Glenn had an audience of 87,000. Other news organizations put the figure at "tens, perhaps hundreds of thousands of people" (NBC News), "more than 100,000 people" (ABC News), and "tens of thousands of people" (NPR).
After President Obama's inauguration, a Slate Explainer column tackled this question: "How Do You Measure a Crowd?"
"Basic arithmetic," Juliet Lapidos writes. "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.
She cites "The [?]-Man March," a 2003 Wired magazine article, in which the director of Boston University's Center for Remote Sensing suggests aerial photography usually provides the most accurate count.