xcrunner56 wrote:
there were lots of turns. however, strava says I ran 0.16 extra miles for for the 6k which seems CRAZY. it would be a bummer if I really did that, ...a bummer if I didn't run that smart.
Part of it might have been poor tangent running. But there's another, potentially big, factor.
GPS is almost always going to show you "ran" a little further. Part of this discrepancy comes from the zig zagging and failure to perfectly run tangents, as you mentioned. But also keep in mind, even if you run the tangents perfectly (which is not possible) the GPS has its own inherent error. Every second or so, it pings your location. But it's not exact, every time. There's slight error. Sometimes it records you a few feet to the left. Next time, it might be right on. A time or two later, it may ping you as being a foot or two, to the right.
I've seen some tracings, where someone is running on a road, then suddenly it shows them 100 feet away from where they are across a lake, then a second later, back on the road. The end result is that even if you're running a perfect straight line (which you're not) it's going to record you as weaving, ever so slightly, sometimes more than slightly. If you measure that weaving line, it's always a little longer than the straight(ish) line you're running, or that recorded by the course measurer.
They try to correct for this in the software, but it's impossible to eliminate it completely. GPS will very closely estimate your time and distance. But it's still an estimate. I've found over the years, that my GPS (Garmin) always says I'm running between 2-5 seconds per mile faster than I am, depending on how many turns there are in the course (more turns = closer to 5 seconds, few turns = closer to 2 seconds). To correct for this, I simply make sure I'm pacing 2-5 seconds quicker than my goal, depending on the course.
Tldr: GPS watches aren't perfect, so always run 2-5 sec/mile faster than you think you need to, for time goal.