Originally Posted by sestivers
CdCf: I am curious as to how you know that your 30+ mile "known" course is the actual distance you're using for your calculation. I'm not trying to be argumentative, I just wonder how you know where your starting line and finish line are on a course that long.
The only idea I have is on a highway with mile post numbers on it, and you start at one mile post sign and end at one 30+ miles down the road.
These can be very inaccurate, depending on where you live and the specific road.
I've driven I-84, along the Columbia River. The total distance between Umatilla & PDX seems pretty close, but in between, I've seen the MP's vary by 2 miles vs my odometer.
As far as GPS, what if you ride in a big circle and end up where you started? It seems to me that they would only be accurate if you rode in a perfectly straight line. Of course I've never owned a GPS, so I could be....