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A few years ago, one foolproof way of saving the battery on you phone was to turn off GPS. You didn't really need it. At most, it was an added convenience in a few apps.

But it's time to turn GPS back on. Your location has become one of the best things about your phone, your smartwatch, and every other connected device you carry. Our tech is learning to adapt to us, nestling into every aspect of our lives so it is more responsive, more useful, and more intuitive. This is awesome, and it's happening because of three things: location, location, location.

Your phone's ability to pinpoint your exact location and use that info to deliver services—a meal, a ride, a tip, a coupon—is reason for excitement. But this world of always-on GPS raises questions about what happens to our data. How much privacy are we willing to surrender? What can these services learn about our activities? What keeps detailed maps of our lives from being sold to the highest bidder? These have been issues as long as we've had cellphones, but they are more pressing than ever.

Follow Me

One frigid and rainy winter day in New York City a few months ago, I sat in Foursquare's SoHo office as CEO Dennis Crowley drew dots on a whiteboard. He was explaining Foursquare's "shapes," the polygonal map of the world Foursquare has built to accurately and automatically locate you. It uses GPS, Wi-Fi, and Bluetooth to determine where you are, then maps your coordinates and sensor readouts to a real place. (There really is good reason your phone tells you to leave Wi-Fi on to improve location accuracy.) Through years of recording this data alongside check-ins, Foursquare can figure out whether you're inside the coffee shop or just walking by, or whether you're on the first floor or the seventh. It doesn't need you to input where you are; it already knows, and can offer you tips, deals, and ads to match. Foursquare is as much a data company as a mayor-maker now.

I moved to San Francisco three weeks ago, and Foursquare's invisible tracking beam has been an amazing asset for learning the city. When I walk into a new restaurant, my phone dings with a suggestion for something to try. When I get off the Muni (inevitably in the wrong place because SF's public transit system is a nightmare hellride of trains that never go where they're supposed to), a Foursquare notification offers a couple of places nearby where I might drown my sorrows in a beer. I'm never stuck, even when I'm lost.

Imagine life without Tinder's deity-like knowledge of who's both ready to hook up and only half a mile away.

Apps used to just provide a record of your whereabouts alongside a photo or a check-in. Now, they're using your location to tailor the experience not just to you, but to right now. Dark Sky, a terrific weather app for iOS, sends you a notification if it's going to rain—and tells you how long it'll be until the first drops fall on your head. If you fire up Snapchat at Coachella, you're going to get different filters and different content, and it's going to be about indie rock, EDM, and casual drug use. Oh, and try to imagine life without Tinder's deity-like knowledge of who's ready to hook up and only half a mile away.

"Context is the new black," says Aparna Chennapragada, head of product for Google Now. The app, built into Android phones, probably is the best example thus far of the power of location: It can do things like dig up your boarding pass when you get to the airport, or remind you about your meeting at exactly the moment you need to leave. "In the past, in the desktop-web world, If I wanted to know something, I'd actually have to go spell the damn thing out,"Chennapragada says. You go to Google.com, type in a bunch of keywords, "and then there's a bunch of links and potentially some useful information that I have to wade through." But Google found that users have certain needs at certain times. And they're pretty predictable. When you're at the airport, you're probably looking for a terminal map, flight times, or a place to eat. When she was at Disney World, Chennapragada remembers wanting to know what rides to go on, "so I don't get FOMO. But what are the wait times? How do I make the decisions on what to do next?" Rather than make you dig for that information, Google Now just shows it to you.

The whole goal of Android Wear or the Apple Watch is to quickly offer information you need, without making you bury your face in a screen for minutes at a time. These quick bursts of useful info appear on the screen automatically. They are the interface. Collecting contextual data like location lets Google, Apple, and others figure out what you're likely to need right now, and deliver it to you front and center.

Powerful location-tracking also is unlocking entirely new kinds of tech. Detour, the new audio-tour app from Groupon founder Andrew Mason, uses your location to lead you on a walk through San Francisco literally step by step, building by building.

"We have Detours where you're walking past a fish shop," Mason says, "and we want to tell you in the two seconds, the three-meter width as you're going past the fish shop, to look at the turtles."

The difference between that and museums, where you walk up to the painting and press a button to hear its story, is the difference between location today and location only a few years ago. "Other than being practical," Mason says, "it's just this surreal, magical thing—you're still aware of the fact that you're interacting with a robot, and it must've known, somehow, exactly where you were. It feels very cool." Hours before we spoke, I was standing on top of a bank building in San Francisco's financial district, in a secret garden Detour's narrator had led me to through hidden doors and shopping malls. It really does feel like magic.

Retracing Steps

Location-tracking lets developers build fast, useful, personalized apps. They're enticing, but they come with tradeoffs: your gadgets and apps maintain a log of where you've been and what you're doing, and more of them than you think are sharing that data with others.

It's going to advertisers, mostly, so they can lure you into the Starbucks a block away or the merch tent at Coachella. It's as creepy as any other targeted marketing, but most of us have come to accept that it comes with the territory. Jennifer Lynch, a senior staff attorney at the Electronic Frontier Foundation, says it goes deeper. Your data might get sold to your credit reporting agency, which wants to know more about you as it determines your credit score. It might go to your insurance company, which is very interested in your whereabouts. It might be subpoenaed by the government, for just about any reason. Maybe none of that is happening. Maybe all of it is. There's really no way for us to know.

Your gadgets now keep a log of where you've been, and more of them than you think are sharing that data with others.

"This information is all collected and stored by a third party," Lynch says, "and once the third party has the data, as the user you don't have any control over it."

Lynch is careful to say this is hardly a new problem. In fact, your phone's been tracking you for as long as you've had a phone—and your data's been all too accessible most of that time. "Law enforcement agencies send subpoenas to cell service providers and try to get access to this information," she says, "which they collect just as a byproduct of offering service." They don't need a warrant, either. "And of course that information is incredibly revealing," she says. Combine it with the other data we're storing in the cloud, she says, and there's little left to the imagination. "It reveals not just where you are, but what you're thinking about, whether you're at the doctor's office, who you're with."

A 2010 Wall Street Journal report found that more than half of popular apps were storing and sharing your information, sometimes to many sources at once. More recently, a Carnegie Mellon University study found users' locations were captured hundreds of times a day, as many as 5,000 times in two weeks. Personally, I have an ESPN soccer app that asks for my location, vaguely threatening to withhold match highlights without it. SoundHound wants to know where I am so it can tell me exactly which coffee shop I was in when I first heard that new Earl Sweatshirt jam. Dropbox, too, for some reason that has to do with uploading photos and locations and other things that make no sense. Your location is valuable data, both to the app makers and to the people who pay them for it.

Regulators and consumers are starting to make noise about this, seeking a stricter standard for collecting and sharing your data. In a few states, law enforcement must now have a warrant to access your phone's location data. But the short version of the story is this: Your exact location is being recorded and stored. It's happening frequently, and by many sources, including a bunch you wouldn't expect. And there's almost no way for you to know what happens next.

So do we fry the GPS chip, turn off Location Services, and give up on some of the coolest, most personal tech currently available? Probably not. After all, it never stopped us from buying phones in the first place. And it's allowing us to develop more intimate relationships with our tech. But just as we've learned to be careful about where we store photos, and where we share personal information, we're going to need to start being more careful about who we trust to follow us wherever we go.