Strange Ideas that just might work

[Pi]

I thought about a distance mesure instrument for runners. GPS doesn't always works in all terrain (forest or high building cities).
So it's like measuring a magnetic field between the 2 shoes that differs because the shoes aren't always at the same distance from one other, because your pace isn't the same all the time, for example when you're running up a hill. Actually that's the most important problem with the Heart Rate Monitor offered by POLAR.
In my way of thinking you could measure the length of every step by the difference of the interference of the magnetic field a bit the same as with a bicycle computer, but you would need a steady magnetic field, So you could calculate by the increasing or decreasing of inference of the magnetic field the length of every pace...