On the afternoon of January 2nd, I set out to have some fun with my new Dual GPS receiver synced to my I-touch. When paired with the Dual receiver (using the MapMyRun app), I have found that on street runs lacking vegetation, the Dual well tracks distances. But today my curiosity had the best of me so I decided to walk 5 laps in Mountain View Park. I had always figured that the loop was about 1/8th of a mile, but what would my GPS show? Mapping the run with the receiver consistently showed that I covered 0.12 miles on each lap and that the 5 lap workout covered 0.62 miles. In the screen capture above, the errors are obvious as each lap mapped differently (this is pretty close to the expected accuracy limitations of the Dual receiver). The tree canopy surrounding the path in Mountain View Park is probably the cause of the inaccuracy of the signals - this can be seen where the line is shown on the interior of the path, especially on the east end of the park.
I then hand digitized the same route on the satellite image in MapMyRun.com (It also draws from the Google Earth images (see photo above). The hand digitized outline of the park showed that I ran 792 feet or 0.15 miles. This error of 0.03 mile per lap extrapolates to a 0.2 mile error short per mile of lap in Mountain View Park.
My next and most likely more accurate test will be taking the receiver to an open track, clear of vegetation. This exercise however does provide a good example of the errors that these units display during any of our heavily forested trail races. Despite the discrepancies, the Dual receiver works quite well and has made my I-touch more functional than I ever dreamed. Not to mention the fun of experimenting with error!
