... the Countryside Alliance and the police agreed on the CA tally of 407,791 marchers. Critics of the Liberty and Livelihood demonstration may accuse the police of bias towards middle class masses in tweed - but the CA put a great deal of organisation into producing an accurate count.
The CA used two teams of four people each to gauge the numbers passing a specific point during a three-minute period. The first group counted the number of people in a row across the width of Whitehall, and the second counted the number of individual people who passed the same point.
Using the average figure from each team of four counters, organisers could multiply the number of people in a row by the number of rows and come up with a fairly accurate count for a three-minute period.
The CA repeated the exercise every half hour and continually liased with police counters at a point about 800 yards down the road. The method still relied on a sympathetic count by police. If there had been a disagreement, the CA, for all its effort, could not claim the final tally was impartial.
We know the capacity of the capitol mall in a static time-frame. We have the time-lapsed video of the march filling Pennsylvania Ave. for 3.5 hours. If we make adjustments for the differences in area and density, then calculate the time from the start of the march to the end point at the capitol, we should have a good estimate. No?
See: http://www.freerepublic.com/focus/news/2338608/posts?page=41#41