“A census data set is a set of geographical polygons, you know, so like the outline of some city district and a number of people in there,” says James Gill, a Facebook engineer who worked on the project. “What we do is we find locations of buildings. To a good approximation, people live in buildings, and not in fields and forest and mountains. So instead of placing the people in the census district everywhere, we only place them where there are buildings.” Aid workers obviously know this intuitively. The maps help them show up more prepared and ensure no one gets missed.