The “Religion for Breakfast” YouTube channel reported on a survey to determine how often people go to church, eliminating the need for self-reporting and the biases that go with it. The video focused on the numbers obtained, but the way the survey was conducted is disturbing and offers a warning about how much information phone applications collect and pass on to third parties.

According to the video, Dr. Devin Pope of the University of Chicago conducted this survey, obtaining information from people without their knowledge or consent. “Dr. Pope analyzed the movement patterns of 2.1 million Americans, tracking visits to religious buildings on their main day of worship. Think of it like instead of asking someone how often they go to the gym, you just check their gym’s key card records. But let’s pause because I know what you’re all thinking. How can he even know how often people go to a place of worship? Isn’t tracking cell phone data kind of — creepy?” (Yes, it is.)

The speaker goes on to explain: “The company used geolocation data from a company called Veraset. They collect de-identified GPS data from apps on people’s phones. Things like the weather app or navigation tools that ask for your location.”

Veraset’s website confirms this is the kind of information it provides, and sometimes it’s even worse. It boasts: “Create custom audiences with machine learning-driven insights about where they live and work.” “Gain insight into trips and routes taken by single devices throughout the course of a day.” Veraset makes the information it collects commercially available.

Anonymization is supposed to protect individuals from being tracked, but it’s not as easy to do that as you may think. Correlating different data items on the same population sometimes allows narrowing them down to a small group or an individual. The information evidently gives location data down to a specific building, a church. Does it do the same for residences? If you spend every night at a certain address, it’s likely that you’re one of the people living there.

ICE probably can buy the same information, and I’m sure they have experts who can use it to improve their idea of where someone they want to abduct is. (Especially after the Tufts incident, the correct word is “abduct.”)

Some commenters on the video suggested that many people may turn off their phones or leave them behind when they go to church, which would make the survey undercount attendance. Be that as it may, it’s an argument that you should turn off location tracking except when you need it.