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I just installed a Specialized Speed Zone Sport Classic cyclocomputer. After calibrating it using the tape measure method, I tested it against a Magellan Sport Track Pro GPS.
When running straight, the speed readings and odometers matched exactly. While making sharp turns, the GPS speed would read lower and after 10 times around a one mile circuit (with six sharp turns) the GPS odometer read 0.3 miles less than the cyclocomputer. This error appears to be due to the one second sampling rate of the GPS. During a turn the GPS is calculating the straight line distance traveled each second rather than the distance along the arc being traveled. In the extreme case, if you were making a complete circle every second, the GPS would calculate a speed and distance of zero.
The error is probably not significant under most conditions but if you need to know accurate distance or average speed over a course with a lot of sharp turns, use a cyclocomputer.
When running straight, the speed readings and odometers matched exactly. While making sharp turns, the GPS speed would read lower and after 10 times around a one mile circuit (with six sharp turns) the GPS odometer read 0.3 miles less than the cyclocomputer. This error appears to be due to the one second sampling rate of the GPS. During a turn the GPS is calculating the straight line distance traveled each second rather than the distance along the arc being traveled. In the extreme case, if you were making a complete circle every second, the GPS would calculate a speed and distance of zero.
The error is probably not significant under most conditions but if you need to know accurate distance or average speed over a course with a lot of sharp turns, use a cyclocomputer.