Garmin Forerunner accuracy

clv101

clv101

clv101

Soldato
Joined
18 Oct 2002
Posts
10,962
Location
Bristol
Over the last year I've run 3 half marathons with my Garmin Forerunner 305. It has measured the following course distances:
Bath 2008 13.23 miles
Bristol 2008 13.23 miles
Bath 2009 13.22 miles

I'm really careful to start and stop just as I cross the timing line, my timings are never more than a few seconds out but the race is meant to be 13 and 7/64 of a mile or 13.109 miles. So the GPS is systematically reading 0.12 miles or 0.9% long.

Is reading ~1% long an accepted issue with GPS? I would put it down to random error but it's been consistent over three races now. Is is due to arm waving!?
 
I'd check the distance of the race (the first mile is the Brighton half this year we 1.2 miles) and I'd also say that over the distance you probably aren't following the exact line they measured.
 
My exact line may be off a bit, but over a tenth of a mile seems too much even if I was on the outside of each bend. The 'error' is also really consistent. I also presume that the official distance between start and finish are pretty much spot on as these are big professional events.
 
My garmin has never read the exact distance of any race. As Asprilla says you'll not be following exactly the line measured and a tenth of a mile is nothing really and easily added by the odd meander along the course. Some of my regular runs can differ by nearly 0.2 of a mile depending on which side of the road I am running i.e. inside or outside of the bend.
 
I usually get it over distance on my 405. If you think about it, you may be running over distance as you could well be weaving through slower runners or not taking the measured racing line.

I would find 13.23 fairly acceptable as it is only about 200 yards over.
 
Yeah, I guess the only thing that surprised me what that each race is virtually the same amount over distance.
 
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