Racing the line – understanding how courses are measured

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 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.

DC Rainmaker:

View Comments (51)

  • What an outstanding explanation you have created for a non-runner like myself. Your graphics and clear wording sure helped me to understand how the marathon can be even more demanding than just running an incredibly long distance.

  • I have been laying out and measuring our local 5k and half-marathon race courses using the official American Track and Field procedure for several years. The procedure, detailed in a 50+ page manual, can be read at the following link:( The official measuring device for certified race courses is a carefully calibrated Jones Counter. The Jones Counter is a mechanical counter that attaches to the front wheel of a bicycle. ((

    I am posting a zoomed-in cropped screen shot from Garmin Connect showing the GPS data points from a street with curves. I was using a Garmin Edge 200 on a bicycle. I rode the course as one would run the tangents. As can be seen, the data points are all over the place and often not even on the street. GPS is not accepted for measuring race courses but it is a great training aid.

    • Thanks Dom, I will check that out. The way that my zoomed-in track showed the severe cutting of corners and curves, one would think that the GPS measured distance would come up short. However, Ray's and your explanations explains what is going on.


    • Anything you look at on Garmin Connect itself is unlikely to display all of the data points; when that site displays, it reduces the number of points for speed. Even if your device has one-second recording on, it won't plot all the points, and you will see chord cuts like those in your image on the screen, which get worse as the overall length of the activity increases.That could well be part of what you're seeing. Try downloading the FIT file and viewing it in something else.

    • Ray, thanks for the surprising info on the Edge 200. I'm thinking of replacing it with an Edge 520 because of the ANT+ sensors that are available for it. Does the Edge 520 display all of the data points when uploaded to Garmin Connect - unlike the Edge 200?

      Thanks so much for your very detailed reviews. I just recently found your website.


    • The Edge 200 uses smart recording, which means it skips data on recording and thus why you see what you see. Interestingly though, it actually doesn't skip data for the ultimate distance displayed on the unit itself - that's tracked correctly.

  • I have several years’ experience laying out and measuring our local 5k and half marathon courses. When I first volunteered for the job, I expected to use a carefully calibrated bike computer to measure the courses. After investigating how race courses are officially measured, I found that the USA Track & Field Organization ( has a 50+ page manual that details the procedure ( The only acceptable methods for certifying race courses are measurement with a steel tape or a Jones Counter ( mounted on a bicycle wheel. Of course, steel tapes are only practical for short straight courses. The Jones Counter is a mechanical counter that measures wheel revolutions. There are approximately 22 "counts" for each wheel revolution - depending on the circumference of the wheel. This results in a resolution of approximately 8 inches per "count". Because of the changing ambient temperature throughout the day and the resulting change in tire pressure, the effective circumference of the tire is always changing and therefore the calibration of the counter changes throughout the day. Consequently, the Jones Counter is calibrated with four runs on a calibration course of around 1000 feet both before and after a course measurement event. The calibration course has previously been laid out by measurement with a steel tape - and corrected for temperature since temperature causes the expansion and contraction of the steel tape. One can imagine that with a 50+ page manual, the course measurement procedure and documentation is very detailed. This all boils down to if a race record has possibly been set on the course, before the record is accepted, USAT&F staff will come out and re-measure the course. If the course is short, the record is not accepted. The course can be a little bit long but it cannot be the least bit short to set a record on.
    One problem that often occurs after a race is that a participant will complain that, according to his GPS, the course length was too long. Race participants seem to accept their GPS measurement as the “gold standard”. A GPS measurement is not accepted for measurement of an official USAT&F race course. I have zoomed in to the maximum on a race course (or a bike route for that matter) upload to Garmin Connect. It is very revealing why a GPS measured route is inaccurate. GPS samples the course at intervals, usually once per second. A lot can happen in one second – especially on a bicycle rolling at 20+ mph on a course with curves and corners in it. My zoomed-in uploads show corners completely cut off. Some of the data points are not even on the course, but off in left or right field. Also, a runner does not run a straight line as he passes other runners, swings over to water stations, etc. Consequently, GPS devices usually measure a course long. GPS is great for training but not for race course measurement. More on GPS accuracy is discussed in the following links:

  • I had an interesting experience with GPS accuracy this past week in the Marine Corps Marathon. My race goal I was trying to hit a reverse split for the Strava/New Balance challenge. I came through the half at 10:10 average pace and was running faster in subsequent miles.

    I ran the tangents much better than the typical runner on the course, and after 20 miles the GPS was reading about 0.2 long. I had my watch on auto-split and knew it would accumulate some distance vs. mile markers over time, but figured it was all relative in terms of my reverse split goal.

    I was pleasantly suprised to see gps mile 22 and 23 come in with 9:37 an 9:21 splits (at constant effort) and figured the reverse split was in the bag until I noticed that the official mile 23 marker didn't turn up until well after gps had hit that distance. Once the mile 23 marker did turn up, I did the math and realized I was quickly falling behind reverse split pace and a sharp temperature jump with clearing clouds at mile 24 put that out of reach for good.

    What I found in retrospect was that the tall buildings in the Crystal City section of the course led to a jagged gps trace that added 10% to those miles and created artificially fast splits for those miles as my legs were running straight down the course. It looks like the last 6 miles of the course contributed roughly 0.5-0.6 of the 0.8 extra on my final 26.9 GPS distance.

    The lesson for me going forward is to use manual splits throughout in courses with tall buildings (e.g. NYC) and that extra GPS distance isn't always uniform across the race or due to tangents. I am wondering if having Glonass activated might have mitigated this a bit?

  • Just to dispel fiction from fact for GPS on phones and watches on this post. And most of all, consumer GPS is just a training aid.

    Note: I rarely use my watch anymore. I like my Mophie battery packs for runs longer than 4 hours. For races, I don't use GPS, the course is measured, I know the distance between aid stations and timing is provided.

    I posted this as a comment in a Runners's World article, "Is Your GPS Watch Accurate on Race Day?"

    Hopefully, this will help with understanding GPS.

    I had a conversation with Mr. Don Cooke, author of, Fun with GPS, and the former Community Maps Product Manager at Esri (where I work). This is what Don says about consumer GPS (watches, smart phone and low cost GPS for cars). I’ve worked in the geospatial field for over 24 years (12 years at ERDAS [now Intergraph] and over 12 at Esri). Maps, geography and mathematics are some of my passions (and degrees).

    1. Most current consumer GPS devices are accurate 3 to 6 meters.

    2. Many consumer devices rely on Wide Area Augmentation System (WAAS). WAAS are a network of ground stations that provide GPS signal corrections, enabling better position accuracy.

    3. Most smartphones ascribe to Assisted GPS (A-GPS). A-GPS is a system used to improve the startup performance, or time-to-first-fix (TTFF), of a GPS. Most watches don’t use A-GPS, resulting in a minute or more for TTFF. I learned that if you pass your hand over the watch (device) during TTFF or there are blocks in a signal shadow (such as a building, car) that interrupts the signal, this causes the signal to start over. Therefore, don’t play with your watch while it’s trying to establish a fix.

    4. Many devices and programs (i.e. Strava) snap the point to a road or trail, thus giving you the illusion of accuracy (depending on the map used to register the imagery or the map itself, and if it matches the USGS map accuracy standards, it’s within about 2.5 meters to 6.5 meters). So, since it’s on the map, doesn’t mean it’s accurate.

    5. Most watches and phones collect points every 1 second or there about. For greater accuracy, if possible, increase collection to 5/sec. However, this drains the battery faster.

    6. Ionosphere disruptions cause signal delay thus adding to inaccuracies. This varies by time of day, month, latitude and hemisphere.

    7. Accuracy depends on the number of satellites fixed with the device.

    What does this mean? It’s a miracle we can even watch our progress, let alone see a GPS track that follows near a road or trail. Consumer GPS devices are great training aids.

    I've used Runtastic for about 5 years. I'm 99% satisfied.

  • An old post, but so true, and so important too, if part of the fun of running is the time you run. It's something that I forgot about in my half marathon last sunday. Which meant I only looked at my pace, knowing that with anything at or under 5:36/km would mean I would run a new PR. I finished with an average pace of 5:35/km. Seconds later, I found out I had still missed my PR by 24 seconds. Disappointing, but mostly just stupid: I should have kept an eye on the total time as well. Turns out I ran not 21.1km but 21.3km. Which did give me a 5:35 pace, but still no new PR.
    Lesson learnt: if you want a new PR, build in a margin of error and keep an eye on the stopwatch, not just your pace.

  • Talking about the way a GPS works ties in with something that I noticed recently. I monitor my rides with an old fashioned Catseye bike computer and Strava running on my (android) phone. What I've noticed is that the speed measurements are very similar on the flat, but diverge on hills, and the steeper the hill, the bigger the difference, with Strava always reporting low. This leads me to believe that the app isn't taking elevation information from the GPS trace. It also means that assuming your max speed is measured at the same moment on both devices, you can do a little trig and work out the gradient of the hill at that point.

  • So, on the subject of USATF course certifications, if a race is advertised as providing a USATF certified course, but it is not listed on the USATF website, how long after the race should I give the organizers before I complain loudly? (I questioned the lack of a certification on file before the race and got nothing.)

    • Ok, so I finally got a response from the race organizers (2 weeks after the race). Basically, "Uh we certified it last year, but had to change it this year. We said on the website the route was subject to change." So it sounds like they didn't even try and certify the course once they were "forced" to change it. Despite changing it months before the event. Not really sure what my recourse is (but I told them I was very unhappy, and to the USATF about it). But I won't be doing any more of their races without a certification, prior to registration.

    • It is *very* common for large races to not finish certifying until a day or two before the event-- mainly because they may not have the course 100% confirmed with local authorities until then. In such cases, I'd give it a month or two before I started getting concerned. The routes usually appear in the USATF certification database eventually.

      Smaller races, I'm not so sure. I've seen quite a few races erroneously claim to have certified courses-- usually because they don't fully understand what "certified" really means. In one or two cases, the race directors have blatantly lied. I remember one race organizer just picking a random certification number and posting it on his site, figuring nobody would bother checking. (He did not reply to an inquiry about it.) One very large first-time race in the mid-Atlantic last November actually had its attemped certification rejected by the USATF, as the measurer did not follow proper procedures. (No surprise that one wound up being a little short.)

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