Jason McGhee

Jason - Chris's group runs!  =)

Hey Michelle, it was so dark last night I could hardly see my feet, but I thought that was you.

The Austin half went better than expected. I was just a couple minutes off my PR despite little training and a very difficult final 3 miles. I felt like I needed a sherpa on one of the hills.

Hey Jason.  That was me that yelled hello last night right before the Bath House!!  How did your run go in Austin?

This is a good explanation of how courses are measured and why your Garmin won't match the race distance from Ken Ashby, who measures and marks all of our races:

As a course measurer, I am often asked about the distance. If done by USAT&F certification procedure (http://www.usatf.org/events/courses/certification/manual/), it is highly accurate. Two wheeled measurements must agree within 0.08%. The wheel calibration is checked before and after on a steel-taped calibration course, and lengthened by 0.1% to insure records are valid. Running on the outside lanes of curves might add another 0.1%. It turns out that present-day GPS technology is not nearly this accurate.

The DRC 15 km (9.32 mile) race course was measured exactly, run correctly, and timed precisely. Your Garmin Forerunner measured a distance of 9.45 miles, which accounts for the 7 sec/mile discrepancy. That is an error of almost 1.5%, which is typical over a 9-10 mile distance (shorter distances vary more).

Garmin customers, myself included, are disappointed to learn about this. Retailers often claim the watches are "very accurate," which means typically within 5%. Here is what the Forerunner 205/305 manual (http://www8.garmin.com/manuals/984_OwnersManual.pdf) says in Product Specifications (page 64):

<  Update Rate: 1/second, continuous
<  GPS Position Accuracy*: < 10m 50%, typical
<  GPS Velocity Accuracy*: < 0.05 m/s
< * Accuracy depends upon view of the sky.
<  99%-clear view; 95%-typical.

In other words, it measures within +/-1% under ideal "clear view" conditions (no trees, buildings or hills on the horizon), but more typically within +/-5%. Note that your 1.5% error is equal to the ideal maximum velocity error of 0.05 m/s at an 8:00-per-mile pace (0.05m/s X 480s/mile X mile/1600m = 1.5%). Also, the Product Warranty (page 71) states, "This product is intended to be used only as a travel aid and must not be used for any purpose requiring precise measurement of direction, distance, location or topography."

An anecdote: My wife purchased us identical Forerunner 201's in 2004. We noticed the measurements would vary greatly from day to day on the same course. One day, as an experiment, I wore both watches side-by-side on the same arm. Throughout my run, the displayed distances and paces were different!

Bottom line: Present-generation GPS signals are too weak to be reliably triangulated by a wrist-sized device in motion under typical conditions. High-accuracy position measurement requires the device be stationary and average many samples over time. High-accuracy instantaneous velocity measurement requires the device move fast enough (auto, plane, fast bicycle) that the position errors are relatively insignificant. You can expect distance and average velocity to measure within +/-1% only under the very best of conditions.

I did. I parked to run with the couch group at 7a and it wasn't bad - I got my usual spot in the first lot. Then when I came back around 945-10a or so, it still wasn't terrible. Hope that helps.