Restore America Rally Attendance Reports - Too High or Too Low?

Estimates for the attendance at Glenn Beck's Restore America rally vary wildly, according to the Washington Post, from Michelle Bachman's "million" through the "over 500,000" trumpeted by Fox News, down to the ninety thousand or so reported by ABC News. They can't all be right, but it's easy to calculate some reasonable upper and lower limits.

How much room do crowds need?
Professor Herbert Jacobs, whose office gave him a birds-eye view of a popular UC Berkeley protest area in the 1960s, came up with some useful data: "A loose crowd, one where each person is an arm's length from the body of his or her nearest neighbors, needs 10 square feet per person. A more tightly packed crowd fills 4.5 square feet per person. A truly scary mob of mosh-pit density would get about 2.5 square feet per person.

To help you visualize this, imagine the crowd densities in a 2-car garage of 400 square feet. A loose crowd is 40 people, a dense crowd is 89 people, and a mosh pit crowd is 160 people in the garage.

How large an area did the Beck rally occupy?
Using Google Maps, crowd photos from Flickr users, and the area calculator at freemaptools.com, I outlined the area occupied by Glenn Beck's rally, running from the steps of the Lincoln Memorial to the base of the Washington Monument. It's about 46 acres, but no one is allowed to wade in the 7.8 acre Reflecting Pool, so there are about 38 acres left for the crowd to use.

Assuming a standing crowd that is evenly distributed, how many people would fit in the 38-acre rally area?

One acre is 43,560 square feet. Therefore 4,356 people in a "loose" crowd will fill an acre. If it is a "dense" crowd, an acre will hold 9,680 people. A 1-acre mosh pit would hold 17,424 people.

Now, let's calculate some possible attendance figures.
If the entire 38-acre rally area was completely full of people standing at a uniform density, loose crowd density means 165,500 people (4356 x 38) were there. A dense crowd would be 367,840 people (9,680 x 38). Jammed into mosh-pit togetherness, you could squeeze 662,112 people into the rally area (17,424 x 38).

If Bachman's million people had been there, each attendee would have had 1.7 square feet of standing room. That's the equivalent of stuffing 235 people into a 2-car garage, which common sense will tell you isn't going to work.

Finally, examine the actual density of the crowd.
Look at the actual crowd photos from various sources. The distribution is not even. The crowd is dense near the front and along the Reflecting Pool where the view was best. It thins out before the crowd reaches the Washington Monument or the sides of the area where view was blocked by the trees.

Also, the crowd is not standing. The pictures show many people sitting in lawn chairs with ample elbow room between the chairs. There is room between the chairs for others to walk. I played with my patio furniture a bit to determine that those seated people were taking a minimum of 12 square feet each. We have to decrease the number of attendees to compensate for the lawn chairs occupants.

The actual number of attendees is far closer to 100,000 than it is to Beck's boast of 300,000 to 600,000. Michelle Bachman's supposed millions are clearly not there.

Sources:
http://www.freemaptools.com/area-calculator.htm
http://www.washingtonpost.com/wp-dyn/content/article/2010/08/28/AR2010082801106_2.html
Flickr.com photos of the rally by several users