A GPS measurement is often not nearly as accurate as a wheel measured course. The normal wrist-held or recreational devices are accurate to anywhere from 3 meters to 10 meters 95% of the time causing them to often report longer distances on an accurate measured course. There are many other issues that prevent a GPS measurement to be as accurate.
There is a tremendous amount of information on the web related to GPS accuracy and very long descriptive accuracy statements made by different manufacturers. One would think that with all the technology of satellites and computers and software algorithms that the GPS is far more accurate than the old fashioned bike measurement method. Think again.
A little background: To determine an object's location, the GPS system must receive a radio signal from at least three satellites, preferably four.. Since each satellite emits a unique signal, the receiver can then match the signal to the satellite and its orbital position. This is called triangulation. Distance from the receiver is then calculated (for each satellite), and from that data, the receiver accurately calculates its geographic position.
What affects the accuracy of a GPS?
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.
Obviously, mountains and clouds cannot be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. The net result can be best described by a study by Michael D. Londe PHD summary below.
"It could be conservatively stated that at best these types of receivers are accurate to 8m to 10 m at 95% confidence. This set of tests has concentrated on Garmin recreational receivers. Tests that have been run on other brands of recreational receivers have yielded similar results."
Your GPS is a great tool for workouts, for figuring out approximately where you are in a race and for elevation and pacing charts.
"So, whereas the phenomenon of GPS technology has added some unique, valuable and fun aspects to running, and races, they are not as accurate at measuring a course as the standard wheel measuring method, especially when the course has lots of turns." Dave McGillivray (Boston Marathon Race Director)
Runners just run....race and have fun. Stop checking your GPS every 200m!!!
Having said that, I've found the two best ways to use a GPS watch:
* For a measure of distance run (as per accuracy described above),
* Using the lap average pace, to tell you how fast you're running averaged out over the distance of the current lap you're timing, eg, set auto-lap to 1km.
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