How does media estimate crowd size?

The number of people in attendance at an event may seem pretty uncontroversial on its face, but it can turn political—just take the comparison between the crowd sizes at Donald Trump's inauguration on January 20, 2017 and those at the Women's March protest the next day. How exactly do people estimate attendance based on pictures of crowds? There's actually a tried-and-true formula, and it's called the Jacobs Method. Watch the video below to learn more about it