This Jan MSNBC article provides some history on crowd counting in DC...
Could 2 million jam into the mall?
Happily for the District of Columbia, the inauguration crowd certainly won’t reach electoral landslide dimensions. But if people jam into the 81 acres of the National Mall between 1st and 14th Streets Northwest at a tight 5 square feet per person, about 700,000 could squeeze in. The open area around the Washington Monument between Constitution and Independence Avenues, back to 17th Street Northwest, could wedge in another 700,000 at the same density. And assuming a looser crowd far back from the inauguration stand on the steps of the Capitol, perhaps another half a million could be milling around in the Mall area in front of the Lincoln Memorial.
Therefore, it is at least physically possible to pack something approaching 2 million Americans into the 2.1-mile stretch between the Capitol steps and Lincoln’s feet.
http://www.msnbc.msn.com/id/28662672//
A method for mob measurement
Even absent publicity-driven pressures to hype the size of a public gathering, no crowd that doesn’t go through a turnstile can be counted without some margin of error. (And as recent election recounts have proved, even counting something as simple as a stack of ballots can have a considerable margin of error.) But some fairly simple math can be used to make defensible estimates of crowd sizes.
The method goes back to the late 1960s and a University of California at Berkeley journalism professor named Herbert Jacobs, whose office was in a tower that overlooked the plaza where students frequently gathered to protest the Vietnam War. The plaza was marked with regular grid lines, which allowed Jacobs to see how many grid squares were filled with students and how many students on average packed into each grid.
After gathering data on numerous demonstrations, Jacobs came up with some rules of thumb that still are used today by those serious about crowd estimation. 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.
The trick, then, is to accurately measure the square feet in the total area occupied by the crowd and divide it by the appropriate figure, depending on assessment of crowd density...
Wow, that’s some good numbers to work with.
I used 1 person/meter squared. Is there anyone who was there that can read through your post and help us come up with a better number for how dense the march was?
Wow, fascinating. After that I was hoping you’d have a number for us. :)