There's a terrific article in today's SFChron about counting the crowd at last week's peace protest, including a detailed description of the counting method and links to download 10 megs of high resolution photos.
Air Flight Service, a company that specializes in high-resolution aerial photography, produced 9-by-9-inch black and white negatives ... [they] then applied a grid pattern to the prints to provide units in which the marchers could be counted.What I love about this is how simple and low-tech the method is. They took pictures. They counted the people in the pictures. We've been doing this kind of thing since at least World War II. Why has it been so difficult to do this for political actions?