I did two runs on much the same course, on consecutive days. The runs were done in Cincinnati, Ohio, by and across the Ohio River. The GPS device was a Garmin Legend HCx, which has a high-sensitivity antenna. The unit was held in my hand, with a constant alignment towards the sky. The only interference was the occasional tree, and the steel bridge girders overhead.
As you can see, the first run track shows me leaving the bridge and “running” in the river. I can assure you that I didn’t jump off the bridge and begin swimming! Also, the track does not get back on the sidewalk until about 300 yards after I exited the bridge.
The Day 2 track stays on the bridge, just as I ran. (The east and west ends of the run are different, due to activity on the east end that prevented me from following the same route I ran on Day 1.)
Runners, don’t believe your GPS is as accurate as a calibrated-bicycle measurement. We have plenty of tracks that show GPS devices are not accurate enough to be relied upon for precise measurements. We use GPS units to create elevation profiles, and so we have a track for every course we measure. We can see from those tracks that GPS track distance varies from our certified distance for almost every course.
Day 1 run:
Day 2 run: