Map coverage: Car GPS models sold in the United States typically include detailed street maps for the entire country, and many include maps for all of North America. And you can buy additional digital maps for various foreign lands. Nearly all of that electronic map data comes from two or three primary sources, so the information is quite similar, regardless of which model you purchase.
Handheld models are less homogeneous. Some can't display maps at all; instead they report your longitude and latitude, with a trail (or "track") of such coordinates to indicate where you've been. Others let you load street maps and topographic maps, and a few can display downloaded satellite images. Maps can be harder to read on the smaller screens that most handheld models have.
3D map view: Most car models can display map data in 3D map view, also called bird's-eye view. As you travel, the GPS map continually updates to show your current position and the surrounding terrain. Using a device that offers only a 2D view is like looking straight down at a 3-square-inch section of a typical paper map. A 3D view gives you a graphical representation of the view out of your windshield, but from an elevation of a couple hundred feet--something like a low-altitude flight simulator. The resulting map is easier and faster to interpret while you are driving.
Turn-by-turn directions: The raison d'être for any automobile GPS unit, turn-by-turn directions prompt you visually and with spoken instructions regarding when to turn right, when to turn left, and when to get on or off a freeway, from starting point to final destination. Today, most GPS units offer text-to-speech functionality, which means that they pronounce street names for you. (For example, instead of telling you only to turn left in 500 feet, the device will instruct you to turn left in 500 feet onto Elm Street.) Text-to-speech used to be a high-end feature, but today it's more common on low-cost devices. You can find some bare-bones units that lack text-to-speech, but it's an extremely handy feature that's worth paying for.
Most units deliver spoken directions through their built-in speaker, which must have sufficient volume and clear sound to overcome vehicle noise. A few models include a short-range FM transmitter for sending voice prompts though your car's audio system.
Most GPS devices have touch screens, where you use an on-screen keyboard to enter an address--usually with text that automatically fills as you enter the letters--and press the Go button. (Do not attempt to do this while you're driving! Set up your trip before turning the ignition key.) Some newer units offer voice-recognition capabilities that allow you to input your destinations by speaking. This feature lets you make changes to your route while you're driving, but it doesn't always work well; don't rely on it.
The GPS device will take some seconds to calculate the route and point you in the correct direction on its digital map. When driving, you must get the directions clearly and in a timely fashion--not too early or too late, but at the appropriate moment so you can safely prepare and execute your change in direction.
POIs: The points-of-interest feature consists of a database of locations and services--gas stations, hotels, parks, coffee shops, and so on--that you may want to consult during your travels. Most automobile GPS units have millions of POIs in their database. But because businesses change even faster than roads do, POI lists tend to be less accurate than GPS street maps. Better POI lists provide phone numbers along with the names and locations of services.
Screen size and type: Screens on automobile GPS models generally range from about 3.5 inches to 7 inches diagonally. Don't be too quick to dismiss a smallish display: A device with a well-designed screen and clear audio directions can work just fine. And models with smaller screens usually cost far less than their big-screen counterparts. Large screens do have obvious advantages, however: Bigger maps, more room for travel data (speed, direction, street labels), and roomier virtual keyboards may justify the extra cost for you. The screens on handheld GPS devices tend to be smaller. You won't find touch screens on many handheld GPS devices because the smaller screens make tapping less efficient; as a result, handhelds often have far more complex controls.
Real-time traffic reports: Some GPS devices have the ability to receive real-time traffic updates; they can come through a wireless data signal or over an FM transmission. Some GPS devices have a built-in receiver for these services, while others will need an add-on receiver. You may be required to pay a subscription fee for the traffic information, too. Traffic tie-ups and construction-delay details appear on your GPS map, enabling you to avoid jams by choosing an alternate route. Some GPS models will automatically reroute you based on the traffic reports.
Lane assistance/realistic road views: If you're driving in unfamiliar territory, the best spoken directions can still be a bit confusing. Fortunately, more GPS devices now have lane assistance, in which the unit tells you which lane you should be in to prepare for upcoming turns and exits. Similarly, many devices offer real-world images of the road--complete with replicas of the actual street signs--that appear near confusing intersections. With a quick glance at the screen, you can be assured that you're following the correct route.
Media card slots: GPS devices store all of their mapping and POI data in one of two ways: on a built-in hard drive, or on flash memory cards, typically microSD cards. Hard-drive models tend to be faster at calculating routes and searching for POIs, but media cards are more durable. Though hard-drive models don't require a media card slot, some include it. The feature can be useful for updating maps and backing up addresses, as well as for taking advantage of the extra multimedia features--such as a music player or photo viewer--on some high-end GPS devices.
Channels: Channels determine the number of satellite signals the GPS can receive simultaneously. Low-cost models typically have 12 channels, while high-end models may accommodate up to 24. Roughly speaking, the greater the number of channels, the better the device's accuracy.
Waypoints: Waypoints are specific geographic locations, described by longitude and latitude, that you have recorded in your GPS unit. Press the waypoint record button on your GPS at a trailhead, and you can always find your way back to your car. Waypoints are rarely used with automobile GPS devices, but they're essential for handheld models. Low-cost units will store relatively few waypoints; high-end models will store hundreds, permit you to manage them within the GPS, and enable you to label them as you wish.
Tracks: Another centerpiece of handheld navigation is the Tracks feature. As you walk or drive though the countryside, your GPS device records and displays a breadcrumb trail of where you've been. This process lets you easily backtrack to your starting point; some models also let you download your track to your PC and view your travels on a topographic map. You can print out the map and the track for a permanent record of your journey. As in the case of waypoints, low-end models offer limited track storage, while high-end models store many and allow you to identify them by adding custom names.