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Timing gate course discrepancy

Yoder

I have a race this weekend, Monumental half marathon. 

I added the race into Connect and the "official" course downloaded to my watch. 

The weird thing is when I added the race, which Connect said was an official race complete with timing gates, the distance said 13.11mi, however the official course shows as 13.31mi.

I've ran this course several times and it ranges from 13.13-13.3mi.

Not sure why there's a discrepancy between the race distance and the official course. 

A company called Ahotu is listed as the company that provided the official event to Garmin, but it seems like the course should be 13.11mi.

Hopefully someone can weigh in on how these courses are obtained. I'd love to try this feature but it's also worrisome seeing the discrepancy. 

  • 0

    How did your race go? Did you get the timing gates while running the course?

  • 0 in reply to spruitp

    Race went great! I didn't trust the timing gate feature enough to try it. 

  • 0 in reply to Yoder

    Oké , I still can’t get it to work. Also the courses from Ohatu didn’t work. 

  • 0

    My understanding is that you're saying that official race distance is 13.11 mi (which is obviously the length of a half marathon), but the course distance in Connect is 13.31 mi.

    I would say the explanation here is that you're never going to get a map or GPS device to measure the exact same distance as a course that's officially certified and manually measured.

    The reasons the course distance isn't exactly correct in Connect are similar to the reasons your GPS device will never measure the exact distance you ran - there are various sources of inaccuracies in both cases.

    e.g. https://support.google.com/maps/thread/78081293/is-measure-distance-accurate 

    It would actually be a lot more suspicious (to me) if the course in Connect (or the activity on my Garmin watch) matched the *exact* distance of the course. The rare times it's been very close for me, I always assumed that it was the result of various errors cancelling themselves out and coincidentally resulting in a number that's close to the "real" distance.

  • 0 in reply to flowstate

    Of course everyone is used to not having your race measure out to be an exact 13.11mi when you're done, but that's not what I'm talking about. 

    A certified course should be the exact distance that is measured running every tangent perfectly. 

    13.31 is more what you'd see if you ran the course on your own and were very sloppy with your tangents. 

    So how can Garmin or any other company say a certified course is almost 1/4mi longer than the actual distance? 

    Something is off and as usual Garmin doesn't care to reply.

  • 0 in reply to Yoder
    Yoder said:
    Something is off and as usual Garmin doesn't care to reply.

    Correct me if I am wrong but there is a difference between the official, measured, distance and the distance a GPS route will provide. At the end of the day any software used to map the route will use mapping data which is not the same as what something lime MapMyRun or Garmin or Strava will use to plot the course.

  • 0

    As others are saying, but perhaps said another way ...

    It is unlikely that the map software is going to measure distance along a path using course tangents to follow the absolute shortest path.  More likely it will follow the distance along the center of the road or path, which is less efficient than tangents.  Official course measurement will be along the shortest possible path, which will use all course tangents.

    Based on that, I would expect the mapped route to be slightly longer than the official course distance.  

    However, this does bring up an interesting, and perhaps important issue/limitations with timing gates.

  • 0 in reply to mark.yarvis

    > However, this does bring up an interesting, and perhaps important issue/limitations with timing gates.

    TL;DR at least the error associated with timing gates is constant, rather than cumulative (as with the error associated with elapsed distance), assuming the software timing gates are placed as precisely as possible, to correspond with real-world physical course measurements. (Not all km "markers" will have a physical counterpart, but hopefully the race organizers know where they're supposed to be, as they're usually marked on digital/paper maps that come with the race, and presumably there's some source data which specifies the "exact" location of the markers.)

    My understanding is that timing gates (as in the software feature) are placed manually on the course map, to coincide with the known positions of real world timing markers.

    I think there will be some inaccuracy regarding the exact position that you're deemed to cross a given timing gate, given that GPS is inherently inaccurate.

    However, unlike the error with measuring course distance (whether on a map or the device as you run the race), the timing gate inaccuracy is not cumulative - the errors won't add up as you pass more and more timing gates. The error associated with passing a timing gate will always be some constant related to the absolute estimated GPS error at any point.

    For example, the precision of smartphone GPS is said to be +/- 5 m. Seems to me that if I run a race with software/GPS timing gates, then whenever I pass a software timing gate (with no corresponding physical marker), the error is still +/- 5 m. So if I run a half marathon (21.1 km) and I pass the 16 km software GPS timing gate (with no corresponding physical marker), I can be fairly confident that I'm between 15.995 and 16.005 metres into race (assuming there's no tall buildings around to mess with the GPS signal).

    Contrast with the traditional method of just measuring cumulative distance via GPS. I've found that on a good day, I get about 0.5% GPS error during races (I just ran a 10 km race which measured 10.05 km on my Garmin). So when my watch says "16.0 km", I can only be confident that I'm somewhere between 15.92 and 16.08 kms int the race. 

  • 0 in reply to Yoder

    > So how can Garmin or any other company say a certified course is almost 1/4mi longer than the actual distance? 

    My point is that Garmin isn't saying the certified course is longer than the real distance. They're saying that the *digital representation of the course* is longer than the real distance. (Again it isn't too different from the fact that if you run the certified course with a Garmin, it will ultimately say that you ran longer - or in some rare cases, shorter - than the real distance.)

    It's no different than if you tried to painstakingly map out a certified course in google maps - you'd probably find that the distance in google maps isn't the same as the certified course distance either.

    It doesn't matter what digital tool you use, this will always be the case, maybe unless someone invents a highly specialized mapping tool that can take super high-resolution course data that's derived from detailed course surveys (such data probably doesn't exist to such a high resolution, I'm guessing).

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