Every run has become a bit of a gadget fest these days. On my wrist is my new Garmin Forerunner 301 GPS tracker and in my pocket is my ipod nano. If I’m running for time, I’ll listen to some up tempo music, if I’m just running for distance, I’ll do a book. The current book is Bill Bryson’s Short History of Nearly Everything – a great book that I have two copies of at home, one in hardback and one in paperback, neither of which I’ve ever got round to reading.
Two weeks ago I ran the Nike 10k in London. There were 3 or 4 races going on in various places, I ran the Battersea Park loop.
Last time I ran with a marked course with the Forerunner, it was the Great North Run. It had me finishing the race a good 1/2 mile before I actually finished. This run was no different. The watch had me down as running 10.4km and me finishing after 47 minutes – I crossed the line at 48m 34s. Motionbased – the website that hosts the graphic I’ve put just above – had the distance at 10.08km – far closer.
These differences bother me just a little – not a lot, it’s about a 4% difference versus distance on the ground so it’s not terrible. After all, who’d have thought a GPS device could fit on your wrist let alone deal with the constant up and down motion of a runner? But, I’ve been trying to figure out the discrepancies, partly so that I know if it’s telling me I’m running faster than I am and partly so that I can understand the distance discrepancies and plan that finishing burst with a little more accuracy.
I figure if it was just GPS errors, they’d cancel out (because I assume that they’re +/- errors so would make one km a bit longer and perhaps the next a bit shorter, evening out over a 10km run). But every run seems a little longer and, consistently, 10km runs seem to come out as 10.4km meaning that I stop running a good 90-120 seconds before I’m supposed to.
After a bit of research, it turns out that there are a few reasons for this:
– Lost signal – where you run under tree cover or between tall buildings
– Elevation – Motionbased looks to map in 3D and measure distance more precisely
– Observations – the watch parses more data as I’m running than it exports to MB
– Algorithms – different, apparently spurious, numbers are thrown away by each
Garmin have just bought Motionbased so perhaps we’ll see some of this sorted out in a new version of the software (the watch plugs in via USB so I’m hoping that I can just download and go). That said, MB works with various devices and I guess they use the same algorithm, so Garmin making it work better doesn’t mean that it would be any better for other devices.
Meanwhile, I need to run the same course a few times and see if the errors are consistent and then I can just add the 1/2km to the distance and adjust for the error in the target speed. Believe me, it’s quite disheartening to know you’re on a 10km loop and think you’re done 90 seconds before you are. On a marathon course, that would have me marked as done around 1km before the end.