Originally Posted by
pdlamb
Say what? You take a path, throw in a few points way off the true path, and it doesn’t add distance?? OK, perhaps your filtering/post-processing knocks that down – but that filtering is at the cost of accuracy going around corners or curves..
You're talking about 2 different things. He means (multipath) errors due to the different paths a GPS signal might take from satellites. There are various smoothing algorithms to deal with those kinds of outliers, but the result is always an approximation of course. You're talking about errors in the track of the bike's path. Your observation is more important than his, to the question of measuring the path length of the bike's travel btw.
The distances interpolated by GPS are also reduced by cutting corners due to the sampling frequency (best case for Garmin is 1 per second I believe).
If anyone is turned off by all the theory and math, it's pretty easy to compare precisions for yourself. Ride the same route a number of times and see how the GPS distance estimate varies, and compare that with how much the wheel sensor distance varies. Look at the measured track if it's available, and how much it varies from your actual path.