Several months ago, in "Error-Free Business Travel" we discussed how Global Positioning Systems (GPS) are revolutionizing business (or pleasure) travel.

The basics of GPS technology can be stated in a single paragraph: You use a small mobile receiver and appropriate software to receive ultra-precise timing signals from a fleet of satellites originally used by the U.S. military, but now available to all users. The GPS software processes the satellite signals to calculate the exact location of the receiver, in latitude and longitude. (There's lots more detailed info available via the above link.)

A GPS unit's real power is unleashed when it's combined with a digital atlas to turn your raw latitude and longitude information into an easy-to-understand location drawn on a map; or better still, to display your position, speed, and direction on a moving street map that scrolls as you drive, giving you a bird's-eye view of your route. A properly equipped GPS unit also can generate instant, on-the-fly, turn-by-turn directions as you drive. Traveling with a street map-equipped GPS is like having an expert navigator in the seat beside you, letting you drive through an unfamiliar area with the accuracy and confidence of a local taxi driver. GPS can make missed turns--and missed meetings--a thing of the past.

GPS units come in all sizes, shapes, and costs. The general concepts that follow apply to all street-mapping GPS units, but the specific details are for one particular model, the Garmin V GPS, because it's one of the world's best-selling portable mapping GPS units. I especially like the Garmin V because it's small enough to be moved easily from vehicle to vehicle via a simple dashboard mount that requires no permanent installation or vehicle alteration. (It's great for temporary use in rental cars, for example.) Its compact size also makes it easy to take with you when you're on foot.

The Garmin V comes with any of several "base maps" permanently stored in the unit's firmware. The version most commonly sold in the U.S. includes basic information on the complete interstate highway system and the major numbered state highways connecting to the interstates. That's a lot, but note that the base map will not help you navigate the smaller, local streets of your city, town, or neighborhood. For that, Garmin's software arm, MapSource, offers "City Select" CDs that provide local street maps for most of North America. Using your PC, you choose one or more regional maps that cover the areas where you'll be driving, and use a special cable to dump the selected local maps into the GPS's user memory.

How much local road info can you load? It depends on how urbanized a region is and how much memory your GPS has (different GPS models come with different amounts of available memory, with about 20 Mbytes available in a basic Garmin V). For example, the entire, mostly rural state of Maine requires only about 1.5 Mbytes of memory; but the maps covering just the greater Boston area of eastern Massachusetts occupy more than 10 Mbytes of memory. Thus, depending on where you plan to drive, you have to be selective about which maps you load, up to your GPS's available memory limits.

The local and base map info work together seamlessly. If you've loaded a detailed map into your GPS's memory, but drive outside the region covered by the stored map, the GPS then reverts to the less-detailed base map. Later, if you re-enter a region for which you've loaded a local map, the unit automatically switches to the more-detailed local map.

But what happens when you drive outside even the base map area? This isn't a theoretical issue: Say you have a U.S.-based GPS, but want to travel in Europe, or vice versa? It can be done: I recently spent two weeks on a GPS-guided driving trip through Europe, totaling some 2,100 miles or 3,400 kilometers, and involving 15 border crossings between and among six countries (France, Germany, Italy, Switzerland, Austria, and Liechtenstein). I used the GPS both for long-distance road navigation and for local, in-city routing; for finding restaurants, hotels, and gas stations; and for tracking down off-the-beaten path scenic spots, too.