Estimating crowd sizes.

Stress that this is a significant issue; consider the following quote from Salon.com:

The Park Police used to use it to make protest-crowd estimates, but the issue became so contentious they got out of the crowd-estimate business entirely, after Louis Farrakhan threatened to sue when the agency said there were only 400,000 people at his 1995 Million Man March. To avoid such threats, Congress banned the Park Police from counting crowds at future demonstrations.
Discuss the following quote taken from an AP article:
Police officials estimated 800,000 people - mostly Shiites but including some Christians - filled the 37-acre lot to hear Nasrallah, forming a sea of waving yellow Hezbollah banners, mixed with a smaller number of Lebanese flags.
Is this estimate reasonable? Note that 1 acre = 43560 ft2. (How can you figure this out? Type the query into Google!) Now use the following rule of thumb:
• 2.5ft2/person: dense crowd
• 5ft2/person: moderately packed crowd
• 10ft2/person: loosely packed crowd
What other methods of crowd counting are there? (This link is to a review of the book "Imponderables - The Solution to Mysteries of Everyday Life" by David Feldman, William Morrow and Co., NY, NY, 1987; the review discusses various methods):
• aerial photography
• eyeballing
• turnstile counts
• indirect methods (weighing trash)

Steve Doig, chairman of Arizona State University's journalism school, established a reputation as a crowd-counting expert while working as a Pulitzer Prize-winning reporter for the Miami Herald.

See also this Wall Street Journal blog on Counting the Tea Party Protesters at the 9/12 demonstrations in 2009.