question: How do they "estimate" how many people were at events?
comment: A photo is taken. Within the photo a fixed size is determines, like 5x5 feet. Within the square the number of people are counted. Then the total area being measured is divided into similar squares and the number of people in 1 square is multiplied by the total number of squares.
passages: A method of determining how many people are at an event is the scientific analysis of photographs, video images, or both. Based on the quality of the image, it is possible to do a physical head count or to estimate attendance based on the density of people within similar areas. This permits the estimation of people over large areas or where parts of the image are obscured. An extensive discussion of how images were used to estimate the number of attendees at the Million Man March can be found at the Center for Remote Sensing at Boston University's website.
  
  Section::::References.
   Section::::Jacobs's Method.
  
  The most common technique for counting crowds at protests and rallies is Jacobs's Method, named for its inventor, Herbert Jacobs. Jacobs method involves dividing the area occupied by a crowd into sections, determining an average number of people in each section, and multiplying by the number of sections occupied. According to a report by "Life's Little Mysteries", technologies sometimes used to assist such estimations include "lasers, satellites, aerial photography, 3-D grid systems, recorded video footage and surveillance balloons, usually tethered several blocks around an event's location and flying 400 to 800 feet (120 to 240 meters) overhead."
   Crowd counting
  
  Crowd counting or crowd estimating is a technique used to count or estimate the number of people in a crowd.
  
  The most direct method is to actually count each person in the crowd. For example, turnstiles are often used to precisely count the number of people entering an event. 
   Three days after the march, Farouk El-Baz, director of the Center for Remote Sensing at Boston University released a controversial estimate of 870,000 people with a margin of error of 25 percent, meaning that the crowd could have been as small as 655,000 or as large as 1.1 million. It later revised that figure to 837,000, with a 20% margin of error (669,600 to 1,004,400). The Park Service never retracted its estimate.
   The "New York Daily News" said crowd counts depended "dramatically on who you ask." Before the rally Beck expected a crowd of 100,000, and he joked during his rally that "I have just gotten word from the media that there is over a thousand people here today." "CBS News'" commissioned AirPhotosLive for the rally's only scientific estimate, which placed attendance at 87,000 plus or minus 9,000.
   At events in streets or a park rather than an enclosed venue, crowd counting is more difficult and less precise. For many events, especially political rallies or protests, the number of people in a crowd carries political significance and count results are controversial. For example, the global protests against the Iraq war had many protests with widely differing counts offered by organizers on one side and the police on the other side. Another memorable incident occurred when Louis Farrakhan threatened to sue the Washington, D.C. Park Police for announcing that only 400,000 people attended the 1995 Million Man March he organized. The National Park Police still estimates crowd size for its own planning purposes, but does not publicly reveal the figures.
   According to an unnamed ESPN producer for "Outside the Lines," in a special dedicated to Gaetjens, upon working with footage believes the number was 18 and attests to 98–99 percent certainty.
   BULLET::::- An estimated more than 1 million attended the funeral of Italian Communist Party leader Enrico Berlinguer on 13 June 1984 in Rome
  
  BULLET::::- An estimated 1 million gathered in London, United Kingdom for the funeral of Diana, Princess of Wales on 6 September 1997.
  
  BULLET::::- An estimated 1 million gathered in London, United Kingdom for the wedding of Prince William and Catherine Middleton on 29 April 2011.
  
  BULLET::::- An estimated 1 million people gathered in The Mall outside Buckingham Palace in London, United Kingdom for the Golden Jubilee celebrations of Elizabeth II on 4 June 2002.
   In 1377 the first true poll tax was levied in which everyone over the age of 14 who was not exempt was required to pay a groat to the Crown. The records taken listed the name and location of everyone who paid the tax and so give an excellent measure of the population at the time, although assumptions need to be made about the proportion of the population who were under 14, generally taken to be around a third.
   Section::::Structure.
  
  Since its inception, HeadCount has been largely volunteer-driven. It fields volunteer teams in most major U.S. cities. Each consists of a trained team leader who runs the local operations, and a cadre of volunteers. These teams then set up tables at concerts through which they register voters and disseminate information related to political issues and upcoming elections. This approach has allowed HeadCount to keep its operating costs low while reaching a very large number of people. In any given year, the organization sets up these tables at 500 to 1,500 concerts or more.
   The official measurement of the course took place at 2 a.m. on 13 June 2012. It was carried out by David Katz, a member of the IAAF Technical Committee, using a bicycle fitted with a Jones Counter. He was accompanied by two other measurers: Hugh Jones of London, who estimated he had been over the course more than 20 times in the 10 years since London first started assembling its bid, and Mike Sandford.
  
  Section::::The course.:Compliance with international rules and testing.
   Visit duration is the amount of time visitors stay in a venue. Through WiFi counting, owners can track the time a person carrying a smartphone entered and left the venue.
  
  Section::::Business metrics.:Bubble map/heat map.
   In the past, attendance at the event was tabulated on daily basis rather than an individual basis. It is unclear whether this practice still exists at EAA Airventure. For example, in 2006 the Oshkosh Northwestern reported that attendance was estimated at 625,000 by the EAA. The estimate was for total attendees each day, so one person attending 7 days would count as 7 attendees. The paper estimated that between 200,000 and 300,000 individuals attended the event. 
   Some media outlets used ranges to report crowd size. "The Washington Examiner", relying on "photographic comparisons to past events" reported attendance as being "well into six figures." "The Daily Telegraph" and McClatchy Newspapers agreed that the crowd was somewhere in the hundreds of thousands.
  
  No estimates were issued by National Park Service, which had ceased making public estimates after rally sponsors from the Million Man March in 1995 threatened court action over official estimates.
  
  Section::::Post-rally response.
  
  Section::::Post-rally response.:Beck's comments.
  
  Section::::Post-rally response.:Beck's comments.:On the rally.
   Seattle's world record was broken on January 31, 2016 in Saskatoon, Saskatchewan Canada, where more than 20,000 participants came to Victoria park to attempt the Guinness World Record. Rick Mercer was one of the participants who came to shoot an episode about Team Canada's Yukigassen team and compete in the world record. Underestimating the number of participants the event ran out of their 8200 wristbands hours before the competition took place. In total 7,681 participants was the official Guinness record and was achieved by the City of Saskatoon thanks to Yukigassen Team Canada
   NBC News and the "New York Post" put attendance at 300,000. NBC reporter Domenico Montanaro tweeted that an "official at top of memorial said 300-325K." Less exact were the "New York Times", which simply called the crowd "enormous", and Fox News Channel, which referred to "strong" turnout and "huge crowds." Though NPR doubted that an accurate estimate was possible, they nonetheless said attendance was in the "tens of thousands", the same estimate of the Associated Press, while ABC News reported "more than 100,000 people" at the rally.
   Following the event, a wide range of crowd estimates was reported by organizers, protesters, and members of the media. The space occupied by the crowd extended from the Capitol lawn down around the National Mall as far as Third Street. The crowd estimates mentioned in many news outlets included "tens of thousands" and "between 60,000 and 75,000", while many sourced the Public Information Officer of the D.C. Fire Department who unofficially estimated there were "in excess of 75,000" people in attendance. The D.C. Fire Department later released a statement that the crowd estimation attributed to them was false and based on an early estimation of a specific area, Freedom Plaza, not the number of participants in the event.
   The classic wisdom-of-the-crowds finding involves point estimation of a continuous quantity. At a 1906 country fair in Plymouth, 800 people participated in a contest to estimate the weight of a slaughtered and dressed ox. Statistician Francis Galton observed that the median guess, 1207 pounds, was accurate within 1% of the true weight of 1198 pounds. This has contributed to the insight in cognitive science that a crowd's individual judgments can be modeled as a probability distribution of responses with the median centered near the true value of the quantity to be estimated.
   Number of people at a crime scene: Different footwear impressions left at a crime scene will indicate more than one person was present at the crimescene.
  
  Approximate height of the wearer: There is statistical correlation between foot/footwear size and height and also stride length and height. The dimensions of the footwear impression and the distance between impressions can be used to provide an estimate of height. 
   Many different technologies are used in people counting devices, such as infrared beams, thermal imaging, computer vision, and WiFi counting. The current fourth generation of people counter utilize stereoscopic imaging to overcome the limitations imposed by previous generations such as shadow issues, high traffic counting, and disruption of patterns on the floor.
  
  Section::::Current technology.:Video counting.
   The opening anecdote relates Francis Galton's surprise that the crowd at a county fair accurately guessed the weight of an ox when their individual guesses were averaged (the average was closer to the ox's true butchered weight than the estimates of most crowd members).
   Police operated a security force for safety and prevention of unexpected collisions at the protest site. The purpose of counting the number was an essential part to approximate numbers of tactical units to be dispatched. Police used the Fermi method. The Fermi method is estimating the number of people per area with an assumption that 5-6 people can fir in 3.3 square meters (1 plung). As such, the police estimate the number of people according to their internal needs, so strictly speaking, it is not the 'official announcement' of the estimated number of people. The size of the gathering is often a public concern, so the media has only made its own estimates available to the public.
   The Domesday Book does not explicitly detail the population of a place but it records that there was 69 households at Great Paxton. There is no consensus about the average size of a household at that time; estimates range from 3.5 to 5.0 people per household. Using these figures then an estimate of the population of Great Paxton in 1086 is that it was within the range of 241 and 345 people.
   BULLET::::- 79th Reading: This is the final location of the Explorer and Classic events. It is also the check-in location for the Classic event and is the main control centre for the Three Towers Hike.
  
  Section::::Event records.
  
  Records do not include times before 2003.
  
  Section::::Event records.:Jubilee Plus.
  
  BULLET::::- Whisky 5 Alpha (5th Woodley Scouts) recorded a time of 04:14 in 2007, when the Scout event rules allowed for older teams.
  
  Section::::Marketing and branding.
   The Domesday Book does not explicitly detail the population of a place but it records that there were 12 households at Hail Weston. There is no consensus about the average size of a household at that time; estimates range from 3.5 to 5.0 people per household. Using these figures then an estimate of the population of Hail Weston in 1086 is that it was within the range of 42 and 60 people.
  