Musing About the Nike Plus

Don just observed a little quirk about the Nike Plus that occurs to me relatively often: I run to a certain point on the horizon, say four miles away. Then I turn around, and at the end of my session have to run about 200 yards past my front door in order for the sensor to register another four miles. Going out is always a shorter distance than coming back. Trust me, I think about this a lot when I'm not more concerned with other pressing issues semi-related to the Spice Girls reunion.
My humanist-based attempt at reason is this: The thing assumes you run somewhat like a metronome over the course of the run. It allows for slight variations, but since it must use an equation to determine how far you go (variable distance =constant pace x constant time) things can screw up if you run with negative splits (which i usually do, and Don certainly did running up and then down a giant hill.) Ironically, if you speed up then you have to run further than you should to get to your goal because the little computer is trying so hard to maintain pace as a relative constant, that the variable distance gets compacted in order to maintain the same balance in the equation (i.e. the nike plus figures you ran for 40 minutes out to get to your halfway point of 4 miles, so it's not going to let you have 4 miles in 34 minutes...it compromises somewhere in between...say 3.9 miles.)

I must say that the difference is usually minimal for me . After ten miles on Monday, I had to run an additional .1 miles, for example. But the fire trails behind Berkeley are a bit...extreme...in many ways. The size of the discrepancy between the two split times usually determines how inaccurate the distance is. I think the Nike Plus still contains so many fantastic features, that you couldn't expect more of something at that price, without GPS or an accelerometer. Our challenges and the goal system alone have done so much to inspire me to keep logging miles.

Anyway, that's my hypothesis. If anyone reading this happens to be married to a chemical engineer or something, or has an otherwise basic grasp of things, then I would like to hear how misguided I really am and get to the root of the dilemma. Perhaps our legs are just longer when we run home.