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I am sure some of you have come across this question before. I do the timing and race results on most of the courses I measure for Certification and on a new 5K and 20K ran this past week-end I had a couple of people come and comment that the 5K was actually 3.2 miles and the 20K was 12.6 miles. And the reason they know this is because they ran with a GPS strapped to their arm. My initial inclination was to tell them to not trust the fancy instrument but I am sure that someone who spent a couple of hundred dollars on the device would not like to hear that. Is there a good response to people who, I'm sure, will start asking this a little more often as time goes on. I want to have an answer that sounds like I know what I am talking about, rather than taking offense to their suggestion that my course is actually too long. Would appreciate any comments.
Original Post
Rick, it's that Southern gentleman in you that keeps you from doing what we'd do here up North - tell them to get a life.

Seriously, I've been doing some testing of the Garmin Forerunner for Running Times, and I find it's fairly accurate, but not totally so.

First, it gets thrown off by trees, wires, tall buildings, which cause it to lose a good signal from the GPS satellites. And even when it doesn't the manufacturer only states it is 99 per cent accurate. I ran a certified 3.05 mile course by the beach, and it came up with 3.10,

You could tell the people 2 things: 1. The system is only 99 per cent accurate, and less so if there's interference.
2. They probably are not running the SPR.

These 2 factors would combine to produce the "overage" you describe.

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