Estimating crowd sizes
It ain’t easy and it costs serious money to do it correctly!
“For an accurate and efficient crowd estimate, the photographs have to be taken at hourly intervals using two systems onboard a fixed-wing aircraft looking straight down: (1) a digital camera with ground resolution equal to the size of an average person from the flying altitude; data from such an imaging system would be fed to a computer to tally the number of pixels with the specific spectral reflectance for a quick but reliable estimate; and (2) a conventional aerial photographic camera with 9×9 inch film to obtain high definition, stereo photographs for post-event analysis and archival purposes. Only then could the National Park Service undertake the very difficult task of estimating crowds at such events.”