How do you figure out how many people are at a large public event?

Question

For example, at the Million Man March, what was the formula to figure out how many people were there?
EX: * # of people per * square foot

Answer 1

a percentage sample was taken.
probably at multiple places and multiple times to determine a "peak" sample.
how does a nurse or doctor know what your heart rate is?
They check for 15 seconds and then mulitply by 4.
They didn't get the actual count for 60 seconds, but a sample and they projected.

Answer 2

Probably. Also I would expect the density (number per square foot) to vary according to the type of event - people are more tightly packed together at a standing-room-only rock concert than at a marching demonstration.

Answer 3

I think they are able to determine by the crowd in the area that they are in. For instance if your block is so many square feet, Or miles long than you can get an account by the square foot.