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DANSHEIL
07-07-2011, 12:30 PM
I ran a 1/2 marathon this weekend. It is supposedly USATF certified. It has been run for many years and this course has been measured as 13.1 miles + in years past. The course is a large loop with one short out and back turn around. There is supposed to be a USATF certification pin at the turning point, although I cannot validate if in fact that is where the turn around was on race day. The Start and Finish are at a permanent structure.

This year, 5 of 5 Garmins (various models) that I know of measured the course significantly short (12.91-12.96). If it was not at 100% of Garmins measuring short, it would be easy to discount. However, with all of them measuring significantly short, it leaves the question was the course short, or was there an environmental factor causing Garmins not to perform well.

Weather was cool at the start and warming quickly through the race. There was a reasonably strong wind. It was not humid, but had been humid in the days leading up to the race. The course has little in the way of tree coverage. It is at sea level, but we all live near there.

No manual mile splits stand out as odd. No Garmin auto mile splits stand out as odd.

Any thoughts on ways in which all Garmins could have measured this course short?

philipshambrook
07-07-2011, 01:57 PM
So what you are saying is that although it is supposedly a certified USAT course you don't know if it still is. Also, you are unsure that the turning point was in the right place. Reality is that you have no idea whether or not you ran a 'certified' course. The accuracy of Garmin devices is generally good for a consumer device. You still have an accuracy between 1.5% and 1%. I'd say that was pretty good.

A number of posts on this in the forum - here's one https://forums.garmin.com/showthread.php?t=10003

DW152
07-07-2011, 02:28 PM
I can't think of any environmental conditions that would consistently cause all GPS measurements to be short at a given time.

Usually GPS measures races a bit long, and that's normal and to be expected. To measure them consistently short is unusual. The difference between 13.1 miles and 12.91 is less than 1.5 percent, which is pretty small, but it's still odd that they were consistently short.

It could be that the race was indeed short, especially with the out-and-back turnaround. There is a 10 miler I've done for almost 10 years now that is certified and well established and has been around for something like 30 or 40 years now. It is also has an out and back section. What is notable about that section of that race is that the turnaround is at the bottom of a hill, around mile 7 or so, when I'm starting to get tired. At that point, I'm not looking forward to running down the hill, only to run back up it. About two years ago, as I was dreading that section, the turnaround seemed to come early. At first I thought I was just feeling better that year, but when I finished my Forerunner showed a slightly short distance. When I got home and compared the track from that year to other years, it was easy to see that they just put the turnaround in the wrong place that year. It was off by a couple hundred meters, and was probably placed there by a well-meaning volunteer who wasn't used to getting up at 0400 and standing by a cone in the middle of the road all morning. At any rate, the race was indeed short that year by a slight amount.