U.S. President Barack Obama was sworn in on Tuesday in Washington. But the number of people who braved the frigid D.C. weather to watch the historic event could have been anywhere between 800,000 and 3 million, depending on who you talk to.
Researchers have projected widely varying figures for the event's attendance, based on satellites circling above the clouds, aerostat balloons tethered blocks away, television coverage of the crowd, and good old-fashioned mathematics calculations.
Steve Doig, a journalism professor at Arizona State University who specializes in crowd counting, said he is estimating there were 800,000 people in attendance, based on a satellite image taken by GeoEye about 40 minutes before the swearing-in ceremony.
"The space-based image is fascinating because all the low-level shots make you think the crowd is much larger. (In the satellite images), you see the very dense clots of people in front of the JumboTrons, but then the wide open spaces elsewhere," Doig said. "I'd still suspect this crowd was larger than the Lyndon Johnson one, which wasn't estimated with the benefit of an image from this excellent viewpoint."
Estimates have put Johnson's inauguration attendance at 1.2 million, but Doig said he thinks that figure is inflated.
With the images, Doig tries to figure out how many people there might be per square foot and then factors in the surface area.
"It's actually fairly simple math, getting the square footage and dividing that by some number of feet per person," he said. "A scary mosh pit is 2.5 square feet per person. That's about as tight as you can pack people, where they can't move--elevator tight."
If people up and down the Mall were crammed that tight, there could have been 2 million, he said.
GeoEye collected a high-resolution image of Washington, D.C., at 11:19 a.m. EST from 423 miles in space, said Mark Brender, GeoEye vice president of marketing and communications.
"There were high, wispy light clouds, but one could clearly see throngs of people, especially gathered around the large JumboTron televisions spread along the National Mall," he said. "The satellite collects imagery at 41 centimeter ground resolution, so one is able to see an object the size of home plate on a baseball diamond."
Satellites owned by Digital Globe also took shots, from 300 miles up following the polar orbit at a speed of about 17,000 miles per hour, said company spokesman Chuck Herring.
Others made estimates based on video images.
"I just watched the event in the American embassy in Abu Dhabi in the United Arab Emirates!!" Farouk El-Baz, a Boston University professor who is considered the leading authority on providing crowd estimates, wrote in an e-mail. "I do not have the pictures yet, but the video images show nearly 3 million people!"
El-Baz explained how he arrived at his figure this way: The area between the steps of the Capitol Building and the Lincoln Memorial is 2.2 miles. The width of the National Mall is half a mile and there is another one mile along the western greens, he said. "If this area is nearly full it can accommodate at least 3 million people," he said.
"Crowd counting is an art," said Curt Westergard, president of Digital Design and Imaging Service, which took photos of the event with 360-degree spherical panoramic cameras attached to balloons bobbing 500 feet above and a few blocks away from the White House. Fiber-optic cables tethered the balloons to a special launch trailer, which transmitted live shots to CNN.
"We're trying to contribute some of the oblique-angle photos of the scene that might see things under trees that satellite photos might miss (or) people standing in alcoves," he said.
The cameras took the shots between 5 a.m. and 10 a.m. EST, when they were forced to shut down due to air space regulations. The balloons, which measure about 12.5 feet in diameter, only rose to 500 feet instead of 800 feet because of issues with President Bush's helicopter, according to Westergard.
Fixed-wing planes and even helicopters usually can be used, but were prohibited from coming near the event for security reasons.
The U.S. National Park Service, threatened with a lawsuit over its crowd estimate for the Million Man March in 1995, stopped doing crowd projections as a matter of policy. But the agency changed its mind for the Obama inauguration, although it won't release a figure until later in the week, according to USA Today.
Imaging technology also was being used to help the U.S. Department of Interior keep track of crowds for security, public safety, and traffic purposes, according to the GIS Cafe Web site. The Interior Department uses a wall-sized display of high-resolution flat-screen, tiled LCD monitors called the "OptIPortal" that displays 35-megapixel aerial imagery, the report said.
(See more satellite images from GeoEye here.)