Originally Posted by
pdlamb
I'm mildly obsessive about my wired computers' distance measurements, perhaps. When I put on a new tire, and again after it's worn a bit, I'll ride up to the top of a nearby ridge and use the surveyed mile markers to measure accuracy. Coasting downhill (so my pedaling doesn't contribute extra distance through swerving), I'll check the odometer against the mile markers for a mile or two (depending on which side of the ridge!), figure out a correction factor to the distance settings, and apply to the computer. It's like your 200' vs. one tire revolution, except on a larger scale. Then, if it was way off, re-check. If I'm within 0.5-1%, I'm golden.
Beware, you can't do this on just any road. Some county roads in Missouri and Kansas, I think they give the good old boys a case of beer, and send them out for a ride. Anywhere a beer can lands, they put another mile marker. U.S. highways are generally reliable in other places I've been, and a few states and even counties appear to have surveyed their roads with great care and accuracy.
I've measured a few walks with a cell phone; the results are usually laughable. If your tablet gets within 2% of the Garmin, it's better than my phone.
The distance markers should be one mile apart on horizontal ground, but if you were going down a hill, the elevation would add some distance too. If you recall Pythagorean Theorem from high school trig, ... ... ... But if the hill is quite shallow, only a percent grade, then no big deal, no correction should be needed.
I understand the desire for great precision here, I am a retired Geological Engineer, I worked with maps and measurements of the earth for a living. But as far as I am concerned measuring one revolution with a tape measure is plenty accurate. After all, if I am within one eighth of an inch when I measure the circumference of rolling my wheel for one revolution, that eighth of an inch error becomes irrelevant when it would take a three quarter inch error to cause a 1 percent error.