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By Matt Fitzgerald
Different types of run speed and distance devices, and different ways of using them, are more or less accurate than others in different running environments. If you’re a casual runner, these differences, while they do have a measurable effect on accuracy, are not worth worrying about. However, if you’re a serious competitive runner who trains with precise target paces for various workout types and wants the greatest possible degree of accuracy in recorded workout data, these differences need to be accounted for and addressed as well as possible.
In normal training on the roads, all of the speed and distance devices from major brands have an acceptable level of accuracy. For example, my Garmin 305 is consistently 99 percent accurate on the roads, which translates to being roughly four seconds per mile off my actual pace.
That said, it’s useful to know exactly how accurate your device is. You can determine your unit’s level of accuracy by using a bike computer to measure off a mile on a stretch of road you frequently run, and then run it to see how your speed and distance device’s measurement compares. Using this method and other similar comparison methods I have found that my Garmin is not really accurate to within +/-1 percent, as you might expect. Rather, it consistently overestimates distance by roughly 1 percent. This knowledge allows me to make a mental correction of the data my device gives me in every road run. For example, it if tells me I ran 18 miles at 6:56 per mile, I know I really ran something closer to 17.82 miles at 7:00 per mile.
Soon after purchasing my Garmin I was disappointed to discover that it is less accurate on the track than it is elsewhere. It overestimates distance by an even greater amount—upwards of 3 percent. The GPS tracking resolution apparently just isn’t adequate to read those tight counterclockwise turns precisely. Since perfect accuracy is paramount in track workouts, the device is essentially useless to me in that environment. Well, it was—until I purchased the footpod accessory that converts the Forerunner from a GPS-based device to an accelerometer-based device. The footpod can be calibrated on the track itself, ensuring almost perfect accuracy there.
The problem I encountered with my Garmin is universally experienced by owners of GPS-based speed and distance devices at the track. So if you make a habit of running there, you should either purchase an accelerometer-based device such as the Suunto t6 or do as I did and convert your device with a footpod.
I have been assured that the performance of GPS-based speed and distance devices on the track will improve in the future. It’s a matter of waiting for a full switchover to a dual frequency GPS platform.
Naturally, you can’t use a GPS-based speed and distance device indoors on a treadmill. Since a properly calibrated treadmill provides accurate speed and distance information, you might wonder why you would even need to use a speed and distance device at all on a treadmill. The reason is data capture. Monitoring and controlling speed and distance is only half the purpose of using a speed and distance device. Capturing data to download onto your training log is the other purpose, and for that you need a speed and distance device.
If you run regularly on a treadmill, purchase either an accelerometer-based speed and distance device or a footpod that converts your GPS-based device for use indoors. This will allow you to capture pace, distance, and possibly also heart rate data throughout your treadmill workouts, just as you do on the track and the roads.
Some speed and distance devices record changes in elevation more accurately than others. This is something you’ll want to consider if you make a habit of running on hilly routes. Accelerometers actually can’t measure elevation changes at all. Accelerometer-based speed and distance devices rely on built-in barometric pressure sensors to measure changes in elevation. Not all accelerometer-based speed and distance devices have built-in barometric pressure sensors, however, and among those that do, some are better than others. The best ones are in high-end units such as the Polar RS800CX.
A good barometric pressure sensor measures elevation changes far more accurately than GPS, which does a terrible job of it. Fortunately, mapping technologies such as TrainingPeaks Ground Control can largely correct for the inaccuracy of elevation change data collect on a run with a GPS-based speed and distance device. Ground Control matches the coordinates of your route as mapped by your device against precise topographical maps of those coordinates to determine the actual elevation changes you experienced during the run. So if you use a GPS-based speed and distance device, be sure to use TrainingPeaks Ground Control too!
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