The comparison between GPS watches and online maps is a tricky one. We tend to take for granted that GPS watches are accurate since they’re what we use every day. And to some extent, they’re truth for us—it doesn’t really matter whether or not they’re precisely accurate because that’s what we think we’re running. And a half mile here or there isn’t important.
However, when I was creating the RunGo directions for the Lick Brook & Treman FLT course for the FLRC Challenge last year, I was starting from @Petorius’s GPS track from running it, which was a bit longer than the believed 13.1 miles. What I found upon zooming all the way in was that his track readings bounced back and forth across the trail constantly, forcing me to smooth point after point to ensure they were all on the trail that appeared on the map. (I’m not sure what RunGo’s base map is, or where it got the trail markings, but they generally matched up with what was on the ground.) By the time I had finished smoothing the entire course to the marked trail, it was back to that 13.1 miles I’d initially estimated from OnTheGoMap. Various other GPSes also got different numbers: for instance:
In other words, my sense is that we can trust OnTheGoMap’s algorithm to do the right thing and come up with an accurate distance. The question is if the lines it’s putting down to track roads and trails match up with what runners will do in reality. Similarly, GPSes tend to fall down when following the exact trail too, due to reception problems from trees and terrain, coupled with tight turns where the GPS may not catch up with you instantly.
Of course, it doesn’t matter much for trails. Even in a race, no one really cares about the precise distance because the terrain and conditions affect times so significantly, and there’s no comparison against other trail races of the same distance. In road or cross-country races, when the goal is to have a course of a specific distance, the only real solution is to wheel the course.
And for training, whatever. Just go with what the GPS watch says—it’s good enough. 