I, like many others, was asking that question throughout the day. Being on the ground it felt like Croke Park times 20. Slate asks how best to reach a figure:
Thanks to advances in aerial digital photography and computer image-processing, it’s now possible to get a fairly exact head count—without a magnifying glass. As Farouk El-Baz of Boston University explained in a 2003 Wired article, the best way to obtain an accurate image is to fly over the assembly at peak time and take a digital photograph (resolution 1 foot per pixel) from 2,000 feet or less. Using satellite images, an Arizona State University professor calculated that about 800,000 people attended the inauguration Tuesday—considerably fewer than the AP estimate (based on photographs and comparison with past events) and less than half the Washington Post number (based primarily on security agencies on the ground).
I think Slate is looking at it from the wrong perspective. To me the core issue is mobile networks, not digital images. Why don’t the US cell network firms, which deployed extra cell towers all over the mall, just release the data on how many people had cell phones in the area and then use this information for some free publicity?
Perhaps certain people like myself might have more than one phone on them, but most won’t. I imagine it would be fairly easy to tell how many people were on the mall by counting the number of active cell phones in the area.