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. 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. Jacobs' Method. The most common technique for counting crowds at protests and rallies is Jacobs' 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."