Comparative Measurement

Mike Sandford provided a nice group comparison of 26 runners using their electronic gizmos in a half marathon.

Group Comparison

The people doing the complaining are true believers in the accuracy of their units. I doubt we will succeed in changing their minds.

"A man convinced against his will
is of the same opinion still."

"My mind's made up. Don't confuse me with facts."

Pete,comparing the Jones counter to a Garmin Forerunner 305 on the Portland Marathon route.

I've just returned from measuring the marathon course, riding the route more as a runner would than as I would when certifying the course. I cut some tangents but not all of them. I used a Jones counter and also carried a Garmin Forerunner 305 watch. Pre ride calibration of the Jones counter on a 1/2 mile course showed an average count of 15356. Start counts 634000, finish counts 1037512 (I DID NOT use the short course prevention factor), so total distance traveled was 26.277 miles. Post calibration average was 15350. The Garmin 305 recorded 26.35 miles. The Garmin was 0.08 miles longer than the Jones counter. Both measurement devices covered the same route at the same time. Actually I was hoping that the Garmin and Jones would be closer, but that was not what my data shows, I wonder why this is the case. So I can only conclude that not only does the Gramin measure longer than the Jones counter, but runners wearing them are not taking the shortest possible route which compounds the error.

I’ve played with Lee’s data and here are some observations:

Certified true length of the course = 42237 meters.

Lee’s measured length “as runners would run” using average constant = 42297.3 meters


Garmin measured length = 42406.2 meters

Number of turns on course = 36 as I counted from the certification map.

Sum of degrees of all turns = 3070 degrees = 8.85 complete circles.

If a runner ran 1 meter outside the SPR on all turns, this adds 53.6 meters to the course. Total distance run would be 42292 meters.

2 meters outside SPR = 42344 meters
3 meters outside SPR = 42398
4 meters outside SPR = 42452

From above I figure that Garmin users ran about 3 meters outside the SPR on turns, or about 10 feet out.

Lee’s measurement looks like the line he took was about 1 meter beyond the SPR. Maybe so, maybe no.

This is pretty speculative, but it does provide agreement between Garmin and the certified length.

I looked at the site for the Garmin Forerunner 305 but did not find any claims for accuracy

This comes out to a difference of 0.25% between the Jones and Garmin measurements, and agrees well with the numerous tests that I have performed and also the ones that Mike and Gene have done.
When care is taken so the same path is followed for the Jones and Garmin measurements, the difference between the two is almost always less than 0.4%, and often a lot less.