Precise mileage and GPS approximation

If you think your smart phone GPS or your Garmin device measure your mileage to the yard, think again. It depends mostly how fast you move and turny or winding your mountain bike trails are. If you ride switchbacks a lot, chances are you'll be short on actual miles and well over your actual speed.

Of course, when the trail is in straight line, the GPS reading can be perfectly accurate, but when going through a curve, the GPS takes points, calculates the distance between these points and gives the length of the chord of each arc instead of the length of the arc itself. This tiny error gets magnified on a fast, curvy course with frequent direction changes.

On such type of trails, a GPS will register a shorter distance than reality. It all has to do with the time points from the GPS satellite, and in the case of a sharp, rounded corner, every second a fast rider will show a 5.55 yard travel at 12 mph, while she will have actually covered. 7.77yard, so I guess I will have the satisfaction of knowing that my mileage is quite conservative on my most winding trails!