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Crowd size in a demonstration is important because organizers tend to use it as evidence of support for their cause. Sunday, which organizers said was when crowd size was at its peak. That number does not take into account marchers who dropped out before or arrived after the moment the photo sequence was shot. Calculating a precise number of protesters for the entire rally is not possible from this survey, but the result is much more accurate than the visual scan method most commonly used by police and organizers. The Chronicle hired the Santa Clara air photography firm Air Flight Service, a company with 20 years of experience in taking photographs for topographical maps for government agencies and private companies. Using a fixed camera mounted in the floor of the plane, the crew made images of the rally from 2,000 feet. The photographs -- taken directly above Market Street and Civic Center Plaza and enlarged -- provide a perspective that allows a discrete count of individuals and a view of the spaces between them, a view that is impossible from ground-level. Both Air Flight Service and The Chronicle examined the photo survey and independently arrived at the estimate of 65,000 marchers at the time the photographs were taken, a figure supported by public transportation statistics. The flight service says its count is accurate within a range of plus or minus 10 percent. This verifiable technique, experts say, could replace the current politically sensitive count totals, whereby organizers tend to provide a high number of participants, and police generally provide a lower number. Come on, that's ridiculous," said Bill Hackwell, spokesman for International ANSWER, one of the groups that organized Sunday's march and rally. At the time the photographs were taken, many people had gathered in the plaza, and the head of the march was at McAllister Street. The tail was near Sansome Street, and no side street was full enough to count, said Jack Barcelona, who runs the air photo firm. Overlaying the photographs with a grid, surveyors from Air Flight Service estimated crowd density in the plaza and along the route. Each grid was evaluated and assigned a density of people, from 10 percent to 100 percent full. This is the first time the firm has used its equipment for crowd estimation. In comparison, both police and rally organizer figures are based on estimates of previous crowd sizes and on eye-level approximations of the event Sunday. To reach the police and protester count of 200,000, more than twice as many people as were photographed by Air Flight Service would have to have left the event before the photos were taken or joined it afterward. PLAZA CAPACITY ESTIMATE Sunday's police estimates started with a calculation of Civic Center Plaza capacity made by the San Francisco Recreation and Park Department in the mid- 1980s, which found the plaza holds a crowd of 43,000 people, said Deputy Chief Suhr. The plaza stayed full throughout the day, even while marchers still filled Market Street, Suhr said. Based on previous estimates of crowd sizes on Market Street, he concluded the roadway and sidewalks had about 100,000 people on them. Other tributary marches and people in side streets pushed the number to between 150,000 and 200,000, he said. The crowd also had pushed the front of the march from First Street to Third Street before it began. However, it did become more densely packed later as marchers continued to arrive. Peace groups based their number on a comparison with a rally Jan. Sunday's protest was similar in size to the march last month, Hackwell said. Police had initially counted the January gathering at 50,000 participants but revised that figure to 150,000 by saying the original number was a count of Civic Center Plaza, not the whole march area. At Sunday's march, Hackwell said, a man stood at Hallidie Plaza to videotape passing marchers on behalf of event organizers. Results from that count were not final Thursday, he said. That number did not account for people who went directly to Civic Center, bypassing Market Street. Over the decades, counting large crowds of demonstrators has been unreliable. During the Vietnam era protests in the 1970s, newspaper editors often would split the difference between protesters' high numbers and police agencies' low estimates and print that figure as the crowd count, according to Todd Gitlin, a professor at Columbia University's journalism school. Recent media practice, however, has been to print both estimates, Gitlin said. Chronicle Executive Editor Phil Bronstein explained why the paper undertook the project: "After hearing concerns from our readers about our accuracy in reporting crowd size in demonstrations, we were determined to come up with a better method to calculate the number of people who turn out for such events. COUNT OF BART RIDERS The Chronicle figure was supported by BART and Golden Gate Transit rider numbers. On Sunday, BART officials estimated that 150,000 people exited the BART system, a figure they later updated to 190,000. By comparison, 23,406 people had exited those same gates the previous Sunday, Feb. Not all riders attended the march and not all marchers rode BART. Figures from Muni, whose trains were full heading into the city, are not available, said Maggie Lynch, Muni spokeswoman. Ferries from Sausalito and Larkspur transported 7,000 people to San Francisco on Sunday. On an average Sunday, 1,500 people make the trip, said Mary Currie, spokeswoman for Golden Gate Transit. The center specializes in photographing large tracts of land from space satellites for various projects such as looking for groundwater in the Sahara. HISTORY OF MISCOUNTS Miscounts of large crowds are common, El-Baz said, even such disparity as calling a crowd of 50,000 a crowd of 200,000. In cities around the world, police estimates of crowd sizes from Saturday's rallies were based on the average number of people in a small area and then expanding that figure to cover the whole route. In London, Metropolitan Police in helicopters counted protesters in the streets based on the density of an average 10-by-10-yard square. That figure was then applied to the length of the march, said Alastair Campbell, police spokesman, in a Chronicle interview. Police estimated 750,000 marchers in the streets and more than 1 million at Hyde Park. In Rome, the Carabinieri, Italy's national police force, said 700,000 people had protested, basing that number on the capacity of piazzas where the demonstration happened and calculating four people per square meter during the march, according to an e-mail from a police spokesman in response to a reporter's questions. Details of how French officials made their estimate were not available.
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