Teachers, scroll down for a quick list of key resources.
- What are some of the chief tools modern geographers, demographers, and data journalists use for estimating crowd size at a non-ticketed event?
- aerial photographs taken by helicopters or satellites.
- weather balloons. “Tethered aerostats” are a specific type of weather balloon permanently tied (tethered) to the ground and take images at a consistent altitude. Tethered aerostats are usually flown from buildings around the event at about 400 to 800 feet (120 to 240 meters) overhead.
- A remote-controlled, spherical panoramic camera affixed to the balloons can capture shots of the crowd from every direction, all at once.
- “A tethered aerostat is superior to a satellite for several reasons, but one of the most important is that it always takes photos below the cloud level.”
- Tethered aerostats are also used to evaluate the size of oil spills and to provide potential views from skyscrapers before they are built.
- 3D maps. These maps are prepared ahead of time, and often overlaid with historical photos of similar events. This map provides a prediction of how crowds may congregate.
- Mechanical Turk. Amazon Mechanical Turk is a crowdsourcing platform that allows individuals or businesses to cheaply and easily access simple “human intelligence tasks” such as counting the number of people in a photograph or video.
- inferences. Indirect evidence of crowd size is important. These inferences may include self-reports and anecdotal data about who went and the use of transportation in the area. (Public transportation and private parking are usually both ticketed. Tickets provide more easily accessed numbers than attendance at the non-ticketed event itself.)
- Why aren’t all these tools available to crowd scientists evaluating the crowd size at U.S. presidential inaugurations?
- For security reasons, airspace around Washington, D.C., is closed during presidential inaugurations.
- How do crowd scientists work “on the ground” to evaluate crowd size?
- Jacobs’s method. Jacobs’s method involves dividing the area occupied by a crowd into sections, usually 100-by-100 feet or 500-by-500 feet. Then, scientists estimate the average number of people in each section, based on algorithms for low-density (one person per 10 square feet, when crowds might have an arms-length between them) and high-density (one person per 4.5 square feet, when they’re shoulder-to-shoulder), and “mosh-pit density” (one person per 2.5 square feet). Finally, these numbers are multiplied by the number of sections occupied.
- physical interaction. Teams walk around the event, counting people in the shade—under awnings, beneath trees, under umbrellas. On-the-ground research is also a great way to estimate the number of children present.
- Crowd scientists need to consider the weather: “If it’s in the winter, we look for the wind breaks, and if it’s in the heat of summer, we look for the shade,” says one expert.
- From both photo-video evidence and on-the-ground reporting, crowd scientists can document how crowds are not uniform and do not fit into neat Jacobs’s method-type grids. During inaugurations, for instance, crowds gather around jumbotrons.
- crowd inspection points. Evaluators set up one or two fixed counting stations near the focal points of an event, and tally the number of people who pass through one or both.
- How do crowd scientists create a “density map” of an event like an inauguration?
- They literally do a head-count. “We sit there literally, head by head, going tick-tick-tick-tick-tick … It’s painful, it’s long, but it’s far more accurate than these algorithms.”
- How does topography influence crowd density maps and estimates?
- The appearance of crowds can vary based on an area’s hills and valleys, no matter how small. “‘If you have people surrounding the Washington Monument—which is on a moderately steep hill—and you look out at a crowd, you’re going to see more people because they’re tilted toward you,’ says a leading crowd scientist. A computer model will correct for those kinds of inaccuracies.”
- Why does crowd size matter?
- In the case of political events, it matters to organizers and participants. In the case of free concerts, it matters to artists and promoters. In both cases, crowd science allows organizers to estimate the number of people their events reached, and how those people accessed the event. (For instance, did attendees crowd the stage or gather around the jumbotrons? This may help in planning for security or technology at future events.)
- Estimating crowd size has a crucial role in security. “If a fire, terrorist attack, stage collapse or other calamity happened at a large event, [one crowd scientist] figures that within 20 minutes he could provide first-responders with the location of the threat and rough estimates of the number of people who might need treatment.”
The Atlantic: How Will We Know Trump’s Inaugural Crowd Size?
Popular Mechanics: The Curious Science of Counting a Crowd
LiveScience: How Is Crowd Size Estimated?