Last week on one of my review posts, someone asked why their Garmin GPS showed a longer distance than the actual course distance of the National Marathon. Of course, there are two possible explanations, but likely only one is the case. The first is that due to a race-day placement issue – a cone, turn or otherwise – was incorrectly executed. But the second and more likely explanation is that you ran longer than the official course distance.
How is that so you might ask? Well, it takes an understanding of how courses are measured. The good news, is this topic is documented in incredible detail on the USATF site (some might say painful detail).
The first step in the process of getting a course certified is to fill out this nifty registration PDF form with USATF (USA Track and Field). However, the real meat of the course certification process is actually measuring the course by a certified measuring person. This is all contained in a sizable 1MB PDF file. And it’s in this document where the gooey details are.
For example, let’s look at a simple two-turn case. In the below scenario, the actual course distance is officially measured 30cm from the curb. The total course also has a .1% error factor (added to it to ensure it’s never short).
(Taken from page 19 of the USATF measuring manual)
Now imagine the typical case where a few thousand (or tens of thousands) of runners are running the course, where will you likely end up? Probably the red line instead. Weaving and swerving around runners.
(Professional MSPaint skillz on top of page 19 from the USATF manual)
So how much extra does this one measly turn add? Well…we’ll get there in a second. The next scenario to look at is the twisting road case.
In this scenario, the official course cuts the corners. But in real life? You’re likely swaying through the full width of the road with a few thousand of your closest friends.
Now, let’s see how this all adds up. I went out to my street and measured a simple turn – just like the first example above.
First I measured the inside of the curve. There were actually two successes there – one in getting the measurement, and two in not getting hit by a car. That would have been a fail.
(Pretty sweet ehh? I’ve actually got a rolling measuring stick. Fun details here.)
And then I measured the outer edge of the curve.
From there, these are what the numbers look like:
Yes, I’ll give you a moment to continue admiring my mad paint abilities.
Ok. Moment over.
Now you say – so what, that’s only 40 feet? Well, next we pull up the course diagram for the National Marathon this year. You can search all certified courses here. From that we can pull the official course map.
Of course, I could count all the turns. But I’m lazy. I happen to know that the National Marathon also listed them all out on their website to make things kinda simple. Each turn had someone there to help runners along. In total, there were 64 turns on this course. They add up pretty quickly, don’t they? They also list them on this map in small print next to each turn.
Now, my math only is for a simple kinda skinny two-lane road. In the case of National, it’s actually a four to six lane road in most cases, plus curbs, making for 2-3 times the lengths I discuss. But we’ll stick with my measurements for the point of this.
So, let’s take 40 feet and multiple it by 64 turns. That’s a total of 2,560 feet. Or .48 miles. Yes – almost HALF A MILE extra!
Oh, but here’s the best part, we haven’t even accounted for all the swerving you do. How many times did you hit up the water stop on the opposite side of the road because it seemed more interesting (yes, I have)? Or go around a pack of runners? Or just switch sides to give a bunch of kids high-fives? Yup, they all add up.
Imagine if you had that extra half a mile back? That’d be probably 5+ minutes for most runners on a marathon. A fair bit, right?
Here’s a quick snippet from MotionBased.com for a bunch of people who have uploaded their runs. I did note there is an interesting pattern that faster runners tended to run less (either due to less congestion, or running a straighter line).
(Green implies the shortest or fasted, and red implies the slowest or longest. Note it’s just automated Excel gradient using the values provided – not saying that 5 hours is bad or anything.)
Finally, there’s actually one additional explanation. Different software programs use different algorithms to determine how far you ran between GPS track points. Your GPS file is actually a bunch of ‘track points’ – like a breadcrumb trail. And software (either on the device itself, or your desktop) interpolates all that and determines how far you ran. Some software is smarter than others, and can remove erroneous data points (for example, going into a tunnel you occasionally get incorrect data points on the exit from the tunnel). So I often see cases where my Garmin on the watch will say one thing, and on Sports Tracks it will say something else. Generally Sports Tracks is smarter though… Obviously if you have a foot-pod based system (i.e. many Polars), then the GPS variance won’t apply – but running the wrong line still does.
So, the point being if you’re off by a little bit, it’s probably not because the course is mis-measured, it’s more than likely you simply ran a little bit extra. 
Think of it as extra training in the bank…
And for those curious – I ran 13.2 miles at the National Half this year. So, relatively close all things considered. But that still would have cost me in the neighborhood of about 40 or so seconds. Win some…lose some.
Ok, only you, the gadget guy, would have a measuring wheel. I only have one because I have to measure out the courses for the kid’s tri I run. Why am I not surprized that you have one??? :0)
Ha, I’m not surprised you have one of those rolly things either
(my old Garmin Forerunner 201 does not like sharp turns and is virtually useless on a track)
Great post… and PAINT?!? awesome.
i too want to know why you have a rolly measuring thing?
Interesting post! I just ran the ING Atlanta half marathon, and my Garmin said the total distance was 13.29 miles. I definitely chalked it up to that weavy phenomenon.
But weaving is important.
There’s that sorority-sponsored beer table at mile 22 that’s always on the opposite side of the street from where you’re running…
Wow, I can’t believe you wrote all that. Very informative. I think I’m pretty good at always trying to run the shortest possible route.
lol…I was totally thinking this same thing the other day! My garmin and map my run were off by a quarter mile…I guessed it had to do with my switching sides of the road (pretty slanty road) and the going wide around the turns.
Thanks!
loved this post! it’s great to see it measured out like you did. i would ahve guessed there was some flexibility in measuring but was totally flabbergasted that one turn had a varying distance of over 40 feet! wowzers!
I think it is hilarious that you have a rolly wheel measurer thingy! I typically gain a bonus mile at the Disney marathon thanks to all of the weaving around and what not.
you may be a better artist than a triathlete!!!! just kidding!!
Thanks for posting this. I’ve often wondered about why all my races have run longer (according to the trusty Garmin). I sorta guessed that it was the weaving and stuff, but was still always a little irritated that if I tracked the time at the exact published race distance, it would’ve been a PR! I’ll have to work on running straighter lines!
Great info to keep in mind!
So honestly, I can’t believe anyone actually thinks their Garmin is that accurate anyway. I mean, mine is routinely off up to a tenth of a miles and sometimes a quarter mile off (in either direction). I mean if Garmin was right, when three of us with Garmins run together then we should all be going off at the same time (when we hit miles). My friends and I have a “we have to run until the last garmin beeps” rule.
Not that your analysis was not enlightening – I didn’t really think corners added up all that much!
And I totally admire those paint skillz. What *can’t* you do?!
(and my Garmin was exactly 13.1 at National, despite having been off by more than a tenth in the middle of the race)
Haha, love the paint skills!
I find the info about the faster vs. slower runners really interesting.
Badass MSPaint Skillz. Part of me wants to see some photoshop renditions, but really, the MSPaint is just more entertaining.
Good info though – Thanks Ray!
How interesting! It would have never occurred to me to get out and measure. My partner is a middle school math teacher and is always looking for (non-boring) ways to show her kids how math applies in the real world. Can’t wait to show her this post!
Excellent post – and bonus points for demonstrating the rolling thing. There was actually a Big Sur winner one year who ended up DQ’d because he took tangents across the curves after the course was funneled down to one traffic lane. He claimed that he was so far ahead that shaving a few extra feet wouldn’t have mattered – but if it was a half-mile or more, it might have made a difference. The second place guy was declared the winner.
Thx so much for the in-depth info…would definitely save few yards running next marathon:-)
Good luck with the Boston!
“Thx. well saved $85 for not buying a footpod:-)
also used SportTracks, a great alternative to Garmin TC. Interestingly for the DC marathon, the total mileage on SportTracks says 26.48 and TC it is 26.73 (both are higher than the marathon total of 26.21 although started the timer right on the starting mat/line). Just curious.”
Hi,
How can I disable the GPS on my Garmin 310XT and manually hit the lap button at every mile marker in a Marathon race? Can that be done on the 310XT?
Thank you,
Run123
I wonder how it would pan out with a road bike race where the racing line is not necessarily the shortest distance. Would they officially measure the course in the middle point of the road, apex to apex or the racing line?
I know this post is old…but I’m thrilled to have found it – I was telling a friend that I ran an extra MILE at the Chicago marathon (according to my Garmin) and that it was becuase I took almost every turn wide. He didn’t think that was possible – but it is!!
There’s another reason the GPS may be off: your GPS is generally only accurate to within 15 to 30 feet (5-10m). So in some cases, those extra few feet may be due to not being in the precise location your GPS thinks it is.
It works fine for airplanes and cars, as the increased distances barely register and precise traveled distance isn’t usually an issue, or even needed. But when your lateral variance over a 100 foot (30m) length is 20 feet (6m), the distance increases by about 2%. So 3.107mi (5k) becomes 3.169mi (5.1k), no weaving or cornering involved.
I totally agree with this. I don’t run with monitors anymore so I will never know. I run with a chronometer and I “lap” my watch when I hit the mile marker (to have my pace after the fact). If there are not mile marker in the race (low key races) then I I’ll never know.
Once I went with all to get my best 5K of the season in one of our Seattle faster courses. At mile 1 I was in 7:20. I pushed the pace…the turn around is not coming. Where is it. I hit the T/A at min 12, this was just impossible. It was a certified course. Everybody came with the story like this post, cutting corners, tangents, or being stopped by crowd. This was not the case. The course marshall put the cone 0.15 miles further. In a 5K the error is extremely noticeable. The race director apologized. As a web site posted: “The error negated the best times for the season.”