Counting the numbers of people in crowds is apparently an increasingly contentious task -given that these estimates are used as a proxy for the strength of feeling behind a range of political causes.
There are already numerous ways to count heads -the most sensible approach seems to me to be to use Amazon’s Mechanical Turk process.
Rather than ask the human operatives who do this work, each to count a whole crowd, today’s invention hands each of them only a small part of an aerial photo of the event (taken by a UAV, if necessary)
Where can you find a collection of willing counters? Send the snapshots to the smartphones of members of the crowd itself (perhaps by displaying a phone number on a hoarding which people in the crowd walk past). They might then enter a draw for some event-relevant prize.
The result would be that each subsection of crowd could be counted by ten of its members, resulting in much higher accuracy.
It could be argued that people on a march might be inclined to overestimate the figures, so this effect could be measured by passing certain calibration shots to distant analysts. A better solution, however, would be to exchange pictures between unrelated events.