ROBERT SIEGEL, HOST:
We're going to take a closer look now at the science of counting crowds. Estimates for the Women's March and for the inauguration have varied widely. And NPR's science correspondent Jon Hamilton says that's not surprising.
JON HAMILTON, BYLINE: Scientists who count crowds say they expected to hear wildly different numbers coming from Washington. Dinesh Manocha of the University of North Carolina at Chapel Hill says if there were just a few people and they aren't packed together, it's as simple as taking a picture and letting a computer count the people. But when you have big crowds like those seen across the country in the past few days, it gets tricky.
DINESH MANOCHA: Especially a crowd of this scale when it is more than a hundred thousand, we just can't estimate right. We just don't have an answer today.
HAMILTON: Manocha says even professional cameras only capture about 40 million pixels. So if there are 1 million people at an event, that leaves just 40 dots to identify each person. Curt Westergard is president of Digital Design & Imaging Service, which is actually trying to make an estimate of attendance at the Women's March. They used their own equipment to take photos.
CURT WESTERGARD: We took our tethered surveillance aerostat balloon that takes up a big cluster of high-resolution cameras.
HAMILTON: But Westergard says he doesn't expect to get a precise figure. Clouds meant the company couldn't supplement their own photos with satellite images. And the number of people changed constantly throughout the day.
WESTERGARD: Our main goal, really, on this is just to ascertain a rough order of magnitude so if somebody says a million versus 100,000, we can easily prove one or the other.
HAMILTON: Westergard says his firm will share whatever it learns.
WESTERGARD: We can and do make all of our data transparent. We put it online. If you don't like what we said, count it yourself and here's the data.
HAMILTON: So anyone can make an estimate. Westergard says the company's headcount should be out by the end of the week. Jon Hamilton, NPR News.
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