At the Census Bureau’s National Processing Center in Jeffersonville, Ind., a team of employees reviews current satellite imagery and compares them to images from 2010.

They label blocks as “passive, active or cloudy”. Passive blocks show no change in housing or new addresses since 2010 while active ones show notable changes and require further office or in-person review. Cloudy literally signifies an image is obscured by clouds.

“The imagery is updated so frequently that sometimes we have bad imagery,” Pollicino says. “Let’s say a storm happens to be passing through; all we get is clouds. So now, we have to review it again.”

The MAF is the Census Bureau’s “crown jewel,” according to Colosi of the Decennial Statistical Studies Division. Much of the information is collected by the Census Bureau but the file also includes data contributed by the US Postal Service and local governments through special partnerships designed to ensure the most up-to-date list of housing units across the country.

The Census Bureau only accepts data from these partners but it does not share its list with them.

“This information is used to invite people to respond to the census,” said Census Bureau Chief of Geography Division Deirdre Dalpiaz Bishop. “So, when we conduct the mailing of our letters and our postcards asking people to respond to the census, it’s using the Geography Division's Master Address File.”

The Census Bureau address team uses specific triggers to determine whether to recheck blocks or areas.

“For example, if the Post Office says that there’s a new address in that block, that would trigger that block, and we would review it again using aerial imagery and see if we can see the new housing unit, like a rooftop, for instance,” Colosi said.