Tracking your progress with GPS technology.

a look at the Internet's distance running message boards reveals that thousands of marathoners are now incorporating the use of personal global positioning system (GPS) devices into their training and racing. Once used exclusively for military applications, in recent years GPS use has become essential for civilian boaters, commonplace for late-model car owners, and increasingly popular among the fleet-footed fitness set.

On a recent Runnersworld.com thread discussing GPS use--a quick scroll of which revealed dozens of entries from distance runners--one Boston marathoner and first-time GPS user wrote, "What was really nice was having the average pace set up on the custom page, which let me know where I was [in relation to my overall time goal]." This cuts to the heart of what runners seem to find valuable: For around $150, the wrist-mounted GPS unit can serve as a dedicated personal trainer.

How it Works

The device tracks set points around the globe and measures its relation to the satellites that send these signals. Most units give you not only your exact location in the form of an electronic map, but elevation, distance, and calorimetric readings as well. Units store roughly two years of performance data. They all include a stopwatch and most will alert you when a specified running speed is reached or when you've fallen below a preset pace. Runners can set a training goal by inputting a desired pace over a certain distance. You may then view a graphic interpretation of your actual pace in relation to the ideal, virtual pace you've preprogrammed.

Another feature of these personal training devices is the ability to learn a course. Once you use the device to track a route, it will remember the route and, on subsequent runs, warn you when turns are coming up. In fact, many runners will obtain by running and then post online a course elevation for others to download to PC, and from there use in the handheld GPS unit. To squeeze in a long run in an unfamiliar area while on vacation or business, a runner can mark his/her hotel as a specific location, chart progress on an intricate, exploratory route, and then follow an "electronic breadcrumb trail" back to the starting point.

A Few Limitations

These impressive features notwithstanding, it's worth taking all personal, handheld GPS measurements with a grain of salt. In more remote marathons, occasionally the unit will not receive a strong enough signal from the satellite to tell you precise coordinates, or what your distance currently is. Though a GPS unit will calculate the lost distance once the signal returns, it will do so on a perfect tangent. This can result in the device underestimating just how far you've run for a given split. Generally, though, some runners notice a tendency for the device to overestimate distance by up to 5%--in situations such as one lap on a 400-meter track, where at no time was there signal loss. The GPS device would likely be more accurate if you ran 400 meters in a straight line; in that sense, out-and-back runs are probably the most accurate. It's highly unlikely that failing to run perfect tangents would result in the addition of .3 miles to a marathon's distance, but many marathoners report just that. If your device reports a total distance of 26.5 miles (or more), it may be slightly overestimating the distance. Keep in mind, however, that the USATF rules on course certification build in a certain minuscule extra distance even after measuring the tangents to ensure that the course is never short.

Marathoner and geographic information systems consultant Paul A. Petersen, MS, says, "Your latitude-longitude coordinates will be a lot more accurate than elevation values. Probably the best way to test the accuracy is to compare it to the x, y and z values of a survey-grade GPS unit. But for practical matters, you could probably just overlay the point and route data onto a topographical map or aerial photo and see how they align. I would imagine that it's actually pretty decent for most training purposes."

Some marathoners loop the GPS device, which resembles an oversized wristwatch, right into a band around their running cap; this may look silly but it improves reception over regular wrist wear. You may then set the unit to alert you with discrete audio signals at certain miles, or when your pace drifts from desired mile pace.

Pedometers vs. GPS Devices

In spite of less-than-perfect accuracy, these devices are leaps and bounds above traditional pedometers. Many runners find that the newer model GPS units regularly deliver distances within .03 miles for the same route on different days. One runner reports, "When I upload the tracks to a photo map, they usually are dead-on with the sidewalks and streets ... I would have killed to get that accuracy back when I was navigating in the U.S. Navy."

Even though pedometers measure distance, they are much less accurate than handheld GPS devices because a GPS unit measures distance totally independent of your stride. If you are running on a hilly course or rugged trail, your stride running uphill will be drastically different than your stride running downhill, which will be drastically different from your stride on flat ground. A pedometer indirectly calculates distances by your stride length and how many strides you take. Thus, if your stride length changes over the duration of a run, the pedometer calculation will no longer be accurate. A GPS device will work no matter what--as long as you have good satellite coverage. Petersen says, "You can bank on good GPS coverage in urban settings, but you may run into minor issues in more ultramarathon terrain, though even in forests they work reasonably well." Unlike a pedometer, the GPS unit will give you on-the-fly velocity readings, as well as save route and waypoints for future reference.

"I would say that the biggest pro to GPS technology is simply knowing what distance you actually ran," says Petersen. "Granted the GPS will be off by a few tenths of a mile, but it will still get you a close estimate. I just wouldn't use it for USATF certification or any other official measurements."

(USATF Road Running Technical Council, www.rrtc.net/book; Garmin International, www.garmin.com; Runner's World Forums, www.forums.runnersworld.com; personal corresp., Paul Petersen, MS, Marathon GIS, Logan, Utah)