How do people get a count for large gatherings such as the protest in Hong Kong, where it’s approximated that 2 million people are gathered
In: Mathematics
It’s a lot of estimating… someone will overlay a grid on a picture of the crowd. count several grids squares, get an average, then count the number of grid squares and there ya go… and decent estimate
Herb Jacobs, Berkeley Journalism professor in the 60s came up with the method that is still used today, though refined. He had an office overlooking the quad, which was marked off in 10×10 grids. He counted people in grids during protests. People at arms length were 10 per grid, closer together, 20 per grid, etc.
[Here.](http://course1.winona.edu/eerrthum/math395/CapstoneReport2.pdf) Works best with an aerial view.
In this particular case, I read in a separate thread that someone had set up a camera on the main street in the earlier protest and used a machine learning algorithm to individually count the precise number of individual people walking along the road.
In more general cases, as mentioned by the other posters, it’s a matter of making an estimate of number of people per square metre and multiplying by the area, and if the crowd is moving, further multiplying by the flow rate.
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