The thing to remember about the sensor is that it is an accelerometer. It measures acceleration and from that calculates distance. More importantly though, it measures acceleration only on its horizontal axis. So, keeping it perfectly parallel to the direction of travel will maximize the accuracy of the horizontal acceleration measurements (and thus maximize the accuracy of the distance calculation) and keeping it perfectly perpendicular to direction of travel will minimize the accuracy of horizontal acceleration measurements. In fact, a sensor that is perpendicular to the ground is actually measuring your vertical acceleration and thus is calculating the total height you've gone (as opposed to total distance) during the run.
With this in mind we can deduce that securing the sensor anywhere but the bottom of the shoe (which gives the it the best chance to be parallel to the direction of travel) will be inherently less accurate. Although calibration will minimize the inaccuracy, a correctly calibrated sensor in the bottom of the shoe will always be more accurate than a correctly calibrated sensor anywhere else not parallel to the direction of travel.
The whole line-up consists of the $60 Amps in-ears and $100 Tracks on-ear headphones, which both also come in slightly souped-up and pricier HD variations at $100 and $130, respectively.
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The thing to remember about the sensor is that it is an accelerometer. It measures acceleration and from that calculates distance. More importantly though, it measures acceleration only on its horizontal axis. So, keeping it perfectly parallel to the direction of travel will maximize the accuracy of the horizontal acceleration measurements (and thus maximize the accuracy of the distance calculation) and keeping it perfectly perpendicular to direction of travel will minimize the accuracy of horizontal acceleration measurements. In fact, a sensor that is perpendicular to the ground is actually measuring your vertical acceleration and thus is calculating the total height you've gone (as opposed to total distance) during the run.
With this in mind we can deduce that securing the sensor anywhere but the bottom of the shoe (which gives the it the best chance to be parallel to the direction of travel) will be inherently less accurate. Although calibration will minimize the inaccuracy, a correctly calibrated sensor in the bottom of the shoe will always be more accurate than a correctly calibrated sensor anywhere else not parallel to the direction of travel.
Just some food for thought.