Wednesday, May 18, 2011

Estimating crowd sizes

There has been rather different estimates of how many people are in this picture:

The CBC report that the police estimated this crowd at 5000 (here); Lifesite claimed 15,000 (here), and the National Post, 20,000 (here).

Crowd estimation isn't too difficult a business. The basic procedure is to multiply the crowd density by the area (see the discussion here, from Obama's inauguration)
"A loose crowd, one where each person is an arm's length from the body of his or her nearest neighbors, needs 10 square feet per person. A more tightly packed crowd fills 4.5 square feet per person. A truly scary mob of mosh-pit density would get about 2.5 square feet per person."
How many square feet? Google Earth pro does the calculation for us.

It looks to me like the center walk falls roughly in the 10-square feet per person, and the two sections on the lawn on either side perhaps half that.

My rough guestimate, then, is
  • 3200 people in the center (32,000 sq. ft. divided by 10 sq. ft. per person)
  • 2550 people in the right-hand wedge (51,000 sq. ft. divided by 20 sq. ft. per person)
  • 2100 people in the left-hand wedge (42,000 sq. ft. divided by 20 sq. ft. per person)
This puts the total at c. 8000 -- though probably less, since the left- and right-hand wedges are probably smaller and less densely occupied that the above estimates suggest. On the whole, it looks to me like the CBC is closer to being accurate than either Lifesite (who seem keen to inflate for reasons that escape me.)