Wrong distance calculation

koplovkoplov Posts: 4 [Apprentice Traveler]
I am connected STRAVA to my TOMTOM sports app so every activity on my watch is uploaded to STRAVA.
On my last run, I saw a big difference between the running distance in the TOMTOM watch (5 KM) and the running distance showed on STRAVA (4.6 KM) on the same activity which was uploaded from the TOMTOM sport app.
I tried to check whether there was a problem with the uploading of the activity so I downloaded the GPX files both from TOMTOM sports and from STRAVA and I saw that the longitude and latitude points are the same.
Then I applied the distance calculation formula on the longitude and latitude points and got that I run 4.6KM and not 5KM.
This means that for some reasons the watch calculate the distance wrong.
Does anyone else encountered this problem?
Does anyone knows how to fix it?

Comments

  • AsimKAsimK Posts: 1 TomTom Employee
    Depending on the website that you use to review your activities, statistics are calculated differently. We upload both our own high level statistics like total distance, time, average pace and calories as well as the GPS trace with all of the sensor data. Some websites respect the TomTom statistics, but others recalculate them based on the raw data, and small differences then occur. There are pros and cons to each method. Some of these websites may have more accurate or up-to-date information about, for example, your weight, which may affect the values for calories burnt during an activity. Another element is that some websites automatically insert pauses when your speed drops below a certain value, which results in a better speed/pace statistic for your activity. As far as Strava is concerned it recalculates the statistics which may result in mismatch of data.
  • koplovkoplov Posts: 4 [Apprentice Traveler]
    I am aware that calculation of some statistics (like calorie burn) can be calculated differently.
    However, I can't see how can you declare that some measured data like time can differ (since my running speed is not close to the speed of light).
    The same goes for distance calculations. Is there 2 different ways to calculate the distance between 2 GPS waypoints?
    Isn't it a relatively simple and known geometry equation of calculating the distance between 2 points on a sphere?
    As I said before, I already applied the formula my self on the GPS waypoints data and received the same distance as in STRAVA.
    By the way, I also checked it with ENdomondo and received the same distance as in STRAVA.
  • tfarabaughtfarabaugh Posts: 15,613 Superusers
    There actually are different ways to measure distance as they are not actually measuring the total distance between all the points. They are going through the entire data set and coming up with your path and calculating the distance of that path, throwing out assumed bad data points and connecting others. The data is essentially a scatter graph, as GPS data points can be up to 30m in any direction of your true position, so one point may be 15m ahead of you and the next second it is 10 m behind you. It makes an assumption that you did not run forward and then suddenly run back. There is a smoothing algorithm to get rid of fluctuations and that is the heart of the differences, as everyone does it differently. Strava in particular has a tendency to understate distance and they also truncate down to one digit. So if your activity was 5.57km they will show 5.5kn, they always truncate and do not round up. Also, if you puse the watch and move during that pause the distance may be lost to some sites as they do nothing with it, while others assume the distance between the points on wither side of the pause. Finally, you have to take vertical distance into consideration, as each site deals differently with that. Two GPS points collected may be 50m apart from one another on the horizontal plain, but if they are on a steep hill might actually be 100m in actual distance (think of the length of the base of a steep triangle versus the length of the slanted side). Each device and site deals with vertical distance differently and some do it better than others (taking into account the vertical change versus assuming you are on the horizontal plane).

    I hope this helped answer your question. If so, please mark it as a solution so others can look for it if they have the same question.
  • tfarabaughtfarabaugh Posts: 15,613 Superusers
    Invincible wrote:
    We upload both our own high level statistics like total distance, time, average pace and calories as well as the GPS trace with all of the sensor data.

    Are you a TomTom employee? Your response (the use of we) makes it sound like it, but your identification tag does not indicate you are an employee.
  • msfurahamsfuraha Posts: 1 [Apprentice Traveler]
    I am having a wrong distance calculation issue too. I've been using my Spark watch for a year now. I run the exact same route 2-3 times a week, and the watch has consistently hit the same distances at the same milestones, until about 2 weeks ago when I noticed that it is now measuring the same route as 0.2 miles longer. It is hitting all the distance markers (1 mile, 2 miles, etc.) earlier than before. And I'm suddenly running faster per mile, even though the time it takes me to run the route is within the same range. Given that I've been running this route for 8 months, this is clearly wrong. How can I fix this?
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