PhotoThere's no denying that GPS is a wonderfully useful tool, but any tool can malfunction, break or simply be misused. And of all the “new” (read: post-1990s) technological tools we modern Americans have integrated into our lives, GPS seems most likely to be catastrophically misunderstood — or at least most likely to generate news headlines about catastrophic misunderstandings.

You know the stories. Remember the Oregon couple last September who spent several days lost in the wilderness after their GPS led them off the Interstate onto an unpaved logging road? Or the Alaskan airport that had to put up barricades, after a flaw in Apple Maps software inspired clueless iPhone users to keep driving onto its runways?

One of our readers, a woman named Wendy, might have inspired such a headline herself, except fortunately, she noticed her GPS' bad advice before following it through to a bad result. On March 25 she wrote to report that her new Garmin worked well when she used it around town, but when she looked into updating its maps, she learned the update would take three hours.

When she took the time to update it, she had difficulties uploading the new map software, and eventually wound up returning the Garmin for a refund after the updated maps gave her incorrect directions. “Thank goodness I was NOT on a trip when I encountered these problems,” Wendy noted.

She said she still has an older Garmin model which has always worked well for her, and she's never updated its maps and now never intends to, for fear of replacing correct data with incorrect.

Not just Garmin

Garmin March 25, 2014, 6:18 p.m.
Consumers rate Garmin
Not that Garmin deserves singling out here; plenty of TomTom users have similar complaints: can't update their maps, or can't trust the maps they have.

More importantly: it's worth remembering that even a GPS whose information is never outdated and always completely 100% accurate (a purely mythical GPS, in other words) can fail you in other ways — especially if the signal from one or more GPS satellites is blocked.

GPS uses the principle of “triangulation” to determine where you are. (As the name suggests, triangulation only requires three satellites, though most commercial GPS systems actually take readings from four or more to ensure greater accuracy.)

Here's an oversimplified explanation of how triangulation works: your GPS reads the signals sent from satellites orbiting the earth and then, by calculating exactly how long it takes the signals from various satellites to reach the device, figures out where it is relative to those satellites, and thus determines your latitude and longitude, usually to within a few feet of accuracy.

Lost signals

But if it can't access signals from any satellites, or from only one or two, then it lacks the data necessary to figure out its location. And if you're driving through tunnels, or even beneath particularly wide systems of overpasses, you can easily lose the signal just when you need it most.

TomTom Portable GPS March 25, 2014, 6:20 p.m.
Consumers rate TomTom Portable GPS
Something I know from personal experience: if ever you drive into Boston via the city's “Big Dig” road-and-tunnel network, there's many places where you must choose whether to turn left or right, to take an exit or stay on the main highway – and these places are underground, where no GPS or smartphone can possibly get a signal. So you need to know where you're going, because your GPS won't.

Also, on various road trips through ultra-scenic parts of the Appalachians, there were areas where my GPS would fail, presumably because the mountains blocked signals from one or more satellites. (I'd speculate this is even worse for Western drivers in the much-taller Rockies, though I haven't tested this myself.)

Even if your GPS has a strong signal, you still need to pay attention, not merely to the GPS but to the road itself; otherwise, you might end up like the Washington State driver who ended up in a lake after the GPS mistook a boat launch for a road, the New Jersey man who drove off the road into a house and blamed his GPS for confusing roadways with residences, or the Pennsylvania woman who ignored both road signs and prevailing traffic patterns, and blamed her GPS for the head-on crash she caused driving north in a southbound lane.

You especially don't want to put your complete faith in GPS in cases where deadlines matter. Suppose you're going to a job interview, and you have the address but don't know how to get there. If you rely on your GPS and it loses its signal or sends you in the wrong direction, you could be late for your interview — which pretty much guarantees you won't get the job.

What to do

So what do you do? If you have the time, do a drive-by the day before, to ensure you know exactly how to reach the place. If not, check other maps of the area, and print or write out a hard copy of the map or directions to take with you.

And if you're the worrywart type, as I am, check more than one online map source. Many years ago, when Internet maps were still a new thing and I lived in the Northeast, I printed an online map to a certain job-interview destination, did a drive-by the afternoon before — and that's how I discovered a certain now-defunct-and-good-riddance online map source had sent me to the Chestnut Street in one town, when I actually wanted the Chestnut Street next town over. (Side note: If you ever hear a New Englander complain “Every freaking municipality in my state uses the exact same street names,” that person is exaggerating – but not as much as you'd think.)

Of course, test drives aren't an option for vacations and long road trips; for that, the best backup for your GPS is keeping a road atlas and a magnetic compass in your car. You can't always get a signal for your GPS or your smartphone, but reading a paper road atlas requires no technological backup at all.

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