How do you know how many people are at a march or rally? It can be an important judgment politically, as at Saturday's rally against the spending cuts.
This BBC News article sheds some interesting light on the subject.
It turns out there are a couple of methods for estimating the number of people in a large crowd. Each has its merits and flaws, but in a nutshell the authorities rely on:
1. Measuring the area of streets where crowds assemble and doing a multiplication according to how densely packed it is. "You apply a rule of thumb that three people per square metre is comfortable and four is like a rock concert," according to a TUC expert.
2. By timing how long it takes for a whole march to pass a particular point.
3. The police get an idea simply from experience. "For example, on New Year's Eve, we know the strip along the Embankment by the London Eye holds a certain number," said a police spokeswoman.
One of my first jobs as a journalist was to report on a demonstration in Norwich against the Poll Tax. This involved counting, as best I could, the number of people involved.
I was flummoxed and slightly in awe of the police officer who was able to tell me confidently that between 800 and 1,000 protestors were present. To me, they looked like so many ants milling around beneath the banners and placards.
It's reassuring to learn that even the experts admit it's not an exact science. The man from the TUC says he would qualify any figure with a warning that it has a 20% margin of error, adding: "The media demand for accurate numbers is an impossible one to meet."
