FWIW, here's the GPS trace of my ride around the Kenosha velodrome, recorded with Strava on my iPhone. as you can see, the trace sucks. It couldn't even match up a single full lap of the track. Not every course I ride looks like this, but this track is especially bad. Other riders using Garmin's had no such issue (so it's not just a bad location for GPS reception).

For some reason, the iPhone GPS kept trying to 'snap' me to the nearby road(s).