A “sensor” is a device that measures a physical quantity and converts it into a signal that can be read by an observer or instrument. Sensors that convert analog information into digital form are the most interesting. The information they collect is easy to store, transmit, and reuse.


Digital sensors are all around — the keyboard on your computer, your cell phone, the surveillance cameras in your office building, and so on.


Lots of good things come from having these sensors around, and the systems they attach to — that’s for sure. But they don’t always serve our interests. Let’s take a look at an example of digital sensing gone wrong.


A colleague of mine recently returned from a business trip, where he engaged in important and sober work. He arrived home late from his trip, and his patient and loving wife, already in bed, engaged him in some conversation. Fairly quickly, she asked him whether he had enjoyed himself at the strip bar (!). My hard‐​working and serious colleague was concerned. Why, on returning to the warm glow of his happy home‐​life, should he be asked this question?


As he tells it, he found himself short on cash one evening, and ducked into the nearest establishment looking for an ATM. The generous purveyors of this … nightclub — who could have known it was something more? — graciously allowed him entry for the few moments it took to get the cash and be on his way.


ATMs are digital sensors. They record usage information and tie it to other details, like location. This is known as “meta‐​data” — information about information, such as where and when a given piece of information was collected.


The ATM transmitted this data and meta‐​data back to my colleague’s bank and, through an online banking system, to his wife. The system identified the ATM as being at “Antics Topless Lounge” or something like that. You can understand the short string of inferences that his caring, truly lovely wife drew when presented with this single item of sensed data.


The reporting of ATM location information is a convenience to those who may have forgotten where they used the ATM, but it’s less welcome to someone whose sweet and lovely life‐​partner might draw unfortunate inferences from ATM use in certain locations. Sensors have social consequences, and they’re not all good.


So I was nonplussed by the cover of the latest issue of Government Technology magazine. It shows the front of a police car, photographed from a low angle to give it a pugnacious look. (Alas, I can’t find the image online.) The car is decked out with lights and sirens, of course, but also with sensors — on the roof and behind the windshield.


“FREEZE FRAME,” says the magazine cover, “License plate scanners extend the reach of local police.” Inside, an article describes how license plate scanning by U.S. police agencies is “the next big thing” for catching stolen cars and locating suspects. But the real benefit, according to the chief of detectives and commanding officer of the Detective Bureau at the Los Angeles Police Department, “comes from the long‐​term value of being able to track vehicles — where they’ve been and what they’ve been doing .…”


Make no mistake: there is value in that, just like there’s value in knowing where you used the ATM. But there’s risk in that, too. It’s not an unalloyed good to give people data about your comings and goings — other than your loving, caring family, of course.


Unlike my colleague and his saintly wife, it’s none of the police’s business where law‐​abiding citizens have been going and what they’ve been doing. When these sensors are used for mass surveillance and not just spotting bad guys, that crosses an important line.


This is not an argument against giving police these sensors. They will be a boon for law enforcement and an aid to our safety and security. But if the back‐​end systems put information about every vehicle’s location into a database for later use, that’s inappropriate surveillance of the law‐​abiding public. Unlike my colleague’s charming, gracious, and forgiving wife, the police shouldn’t be in a position to ask us whether we enjoyed ourselves at the strip bar.