They scrutinized photos taken from helicopters and tall buildings. They measured how tightly people were packed along Broad Street and the Benjamin Franklin Parkway. They crunched the numbers.
The result: An estimated total of close to 700,000 Eagles faithful attended Thursday's parade, according to a team of crowd-safety experts at Manchester Metropolitan University in England.
That is well shy of the two million figure that was tossed around in some unofficial pre-parade projections — and the true number may be even lower, because some parade-goers were likely counted twice (though people watching the event from indoor vantage points would not be included).
The event was a moving target spanning the course of several hours, so the researchers broke the problem into three sections: adding the totals from Broad Street between Pattison Avenue and City Hall; from City Hall to Logan Square; and from Logan Square northwest to the Philadelphia Museum of Art.
But nevertheless, it was a massive number of people who bleed green — joining the crowds for the 2015 papal visit and the 2008 Phillies parade among the largest throngs in city history.
Manchester Metropolitan's crowd-counters included seven graduate students in several countries, overseen by professor of crowd science G. Keith Still and colleague Marcel Altenburg, a former German army captain who advises the planners of marathons and other mass events.
By 11:30 a.m. on Logan Square, before the caravan of Eagles players passed by, spectator density was as high as 5.5 people per square meter, Altenburg said. A square meter is roughly the size of a large card table, so imagine standing on that with 4.5 of your fellow Philadelphians.
"This is a high density," Altenburg said after looking at aerial photos emailed by the Inquirer and Daily News. "And it looks cold."
Estimating crowd size is not just an academic exercise for the Manchester Metropolitan team. Their primary goal is safety.
With a Ph.D. in interdisciplinary mathematics, Still has developed sophisticated software to simulate and model the flow of people. He regularly advises emergency planning officials and also has been called upon to testify in legal proceedings. It is not so much the raw number of people that poses a risk of injury, but how tightly they are packed in, the dimensions of entrance and exit points, and the speed at which people are moving through.
Nevertheless, the researchers realize that sheer crowd totals are of interest for a popular audience, and they were happy to undertake a rough count of those who attended the Eagles parade.
Government agencies, on the other hand, shy away from proclaiming official crowd numbers, as the topic can be politically radioactive. Witness the partisan bickering over the relative crowd sizes for the inaugurations of Presidents Trump and Obama. Likewise Mayor Kenney's office steered clear of that landmine for Thursday's parade.
At the Inquirer and Daily News' request, Still undertook a similar effort for the 2015 papal Mass, estimating that 142,000 people were crowded onto Ben Franklin Parkway west of Logan Square. Far higher guesstimates — of up to 800,000 — were widely circulated by the public. Available photos showed only the western end of the roadway where the highest concentration of people was gathered, so Still and his professional counters limited themselves to that hard evidence.
For the 2008 parade to celebrate the Phillies' victory in the World Series, there were no scientific counts taken.
The Manchester Metropolitan team took the Eagles counting project seriously, using digital maps to calculate the areas of the various stretches of pavement where fans were gathered.
As for the unofficial pre-parade projections of at least two million, Altenburg said there simply was not enough room for that kind of crowd, large though it may have seemed to those on the ground.
"You don't have the real estate for this," he said.
In addition to Altenburg and Still, team members included Jim Aughney, Bert Bruyninckx, Benjamin Nigel Cowcill, Eric Kant, Ise Murphy, Jade Margaret Patel, and Mark Whitten — industry professionals with more than 100 years of event experience combined.
Not all the team members are very familiar with American football. But Altenburg, who regularly advises planners of the Chicago Marathon, likes going to Bears games.