Forums >Gears and Wears>"In GPS We Trust"
an amazing likeness
As the racing season picks up, thought it might be a good time to post this article again...
link to full article
"Runner threatens law suit over course measurement disagreement. (True Story)Picture yourself running your best half marathon ever, at least according to your GPS you are. You can hear the crowds welcoming now, and you are dreaming of a personal best at just under 1 hour and 44 minutes. But your time comes and goes and still you are running towards the 13 mile mark. How could this be? Your GPS is state of the art and accurate to the nth degree. Your GPS just said you finished 13.1 miles. You paid almost $200 for this device, it has to be accurate! You grit your teeth and finish strong, but your personal best has long passed by. Where's the race director you ask? This course is SO LONG! ! ! I logged 13.3 miles on my GPS.After a fashion, you lodge your complaint and go home, but later you think again. I paid my race fee it should be the correct distance, why should race directors get away with this. I am so sure my GPS is accurate! After getting an email message from the Race Director that the course is indeed sanctioned by the USATF and certified you are still not convinced. You blast the race director and threaten to sue for the race fee, you even call the race timer up and instruct him to correct your time to the “real” time you ran for the 13.1 mile distance. (Real story)Which is more accurate: A certified course that is wheel measured, or your GPS? Or in truth, how far do I really have to run before I can see the Beer tent?"
Acceptable at a dance, invaluable in a shipwreck.
Yeah... what the article says.
Interestingly (well maybe interesting to some) is that you can make a pretty accurate route measuring device using a bicycle and a cheap cycle computer. The trick is to be really careful about how you calibrate it:
Pump the tyres up good and hard. Find a smooth flat piece of road (you only need a short space). Put a dab of paint on the tyre (the one where the magnet for the cycle computer is fixed). Ride straight for a few 10s of metres. Then carefully measure the space between the paint dabs once you've got up to speed. The cycle computer has a setting for the diameter of the wheel... this is the figure to use.
Now you have a pretty accurate measuring device... as long as the tyre pressure doesn't change much and the weight on the bike doesn't change much.
This won't been good enough for certified course, but likely to be considerably more accurate than measuring with a gps.
Yeah... what the article says. Interestingly (well maybe interesting to some) is that you can make a pretty accurate route measuring device using a bicycle and a cheap cycle computer. The trick is to be really careful about how you calibrate it: Pump the tyres up good and hard. Find a smooth flat piece of road (you only need a short space). Put a dab of paint on the tyre (the one where the magnet for the cycle computer is fixed). Ride straight for a few 10s of metres. Then carefully measure the space between the paint dabs once you've got up to speed. The cycle computer has a setting for the diameter of the wheel... this is the figure to use. Now you have a pretty accurate measuring device... as long as the tyre pressure doesn't change much and the weight on the bike doesn't change much. This won't been good enough for certified course, but likely to be considerably more accurate than measuring with a gps.
You need to adjust the distance between paint dabs (circumference) to the diameter of the wheel by dividing by pi.
Err right. Actually I think they usually ask for the circumference, but check which thing the machine wants.
And add a correction factor for when you've leaned the bike off vertical.
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The two aren't mutually exclusive. The course could be accurately measured, and the GPS could be accurate as well. The runner may have actually run slightly longer due to cornering, weaving, etc.
Janie, today I quit my job. And then I told my boss to go f*** himself, and then I blackmailed him for almost sixty thousand dollars. Pass the asparagus.
My daughter and I both wear gps watches. They rarely beep together (at each mile). She hates it when mine beeps first but is just fine with her's beeping first...
That's true, but in practice the gps is not accurate (or at least, not reliably accurate). If you have access to a running track try running the 100m straight a few times and see what distance the gps gives you. In general it'll be different each time and may well be a few % out.
If you run 400m on the track it tends to be worse than on the straight.
Mostly this doesn't really matter for pace/distance during training purposes - the gps is close enough. But if you're trying to run a particular time/pace in a race then you should be careful about relying on your gps for pacing information. Getting 35km into a marathon and then realising you've averaged 5 seconds per km slower than you thought could well be a problem.
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I've found for most races my GPS is off the advertised distance for a pretty consistent amount. My GPS usually reads marathons at 26.4-26.6 miles. I just consider this for the pace I want to target, and check as the course goes how far off I am getting from the mile markers so I know what the GPS will say at the end by usually the halfway point.
I was at a marathon a few weeks ago where there was a group complaining about how far off pace the pacer must be, because it didn't match their GPS watches. I told them I didn't think so, and to check their time at the halfway point compared to their goal time in a second when we passed it. Sure enough, they were behind.
In my marathon last fall, my GPS measured 26.54mi. That's a 7sec/mi pace difference v. the actual 26.2mi measured distance. I don't doubt I ran >26.2mi, what with crowds, non-tangents and everything. It was a good lesson in what numbers to use in assessing target race pace.
Garmin nazis. Pacers HAAAATE this. And it happens at almost every race these days.
And this is the main reason why I won't run with pace groups, even on days where I'm feeling particularly sociable.
I've got a fever...
In my marathon last fall, my GPS measured 26.54mi. That's a 7sec/mi pace difference v. the actual 26.2mi measured distance.
That's a differential of 1.23%. In the grand schemes of things, that's not much of a difference. There's no such thing as a perfect measurement. Uncertainty is woven into the fabric of the universe.
BTW, I wish all GPS running devices shipped with a copy of this picture so that runners would have an idea of what they are actually measuring:
On your deathbed, you won't wish that you'd spent more time at the office. But you will wish that you'd spent more time running. Because if you had, you wouldn't be on your deathbed.
Fast is better than long
Stolen from the competitor.nashville site who stole it too.
I think the picture posted by jeffgoblue and this fairly well explain the differences.
April 7, 2008 – posted in Ask Garmin, Into Sports, Jake’s Journal, On the Trail, Peg’s Posts,
A Forerunner customer who recently ran the Shamrock Shuffle 8k in Chicago asked us why the distance shown on her Forerunner varied from the official race distance of 5 miles … or 4.9709695379 if you want to be ultra precise.
If the distance shown on your Forerunner is slightly more than the official race distance, it just means you weren’t cutting corners—quite literally. According to the IAAF (International Association of Athletics Federation) course measurement guidelines (pg. 20), a road race course is defined by the shortest possible route a runner could take without being disqualified. For most races, a certified measurer rides the course on a bike, staying near the curb and taking every available tangent. This ensures that all runners will run at least the declared race distance.
After the race, if Forerunner shows that you ran a little farther, this just accounts for extra steps you took to run around others participants, hit a water stop or stay to the middle or outside lane.
Read more: http://nashville.competitor.com/event-info/course/#ixzz1HR9CdpW2
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