Why do Strava segments move?

Red17

Über Member
Location
South London
Noticed on a couple of Strava segments that I like to use to monitor my times on that the end of the segment has moved by 50 yards or so - one now takes me onto a roundabout (used to stop short), and another now finishes round the corner from where it did before.

Any ideas why this is happening - is the earth going out of orbit or something?
 

MiK1138

Veteran
Location
Glasgow
GDPR innit! I can only guess that the creator of the segment has altered it
 
Is this when you use live segments?. I would say it is your GPS reciever being inaccurate or something at the time. I had lined up a Strava section this morning on live segments that I had not set before , that finishes just before a T junction according to the section map on Strava. As I sped up to it my Garmin 1000 was telling me 500ft to go which was beyond the T junction, I hit the brakes thinking mental strava section that finishes after a T junction. It only told me I had completed once I was onto the main road, it told me my time was 3-4 secs off my best. After completing the ride Strava gave me a PR so looks like a fault with my garmin 1000 positioning.
 

wajc

Über Member
Noticed on a couple of Strava segments that I like to use to monitor my times on that the end of the segment has moved by 50 yards or so - one now takes me onto a roundabout (used to stop short), and another now finishes round the corner from where it did before.

Any ideas why this is happening - is the earth going out of orbit or something?
It could be one of two things

1) You've basically hit on the fact that Strava segment length can vary for every rider and can vary every time a rider rides that segment.

This is most evident if you ride side by side with another rider through the whole segment and record different times and speeds from each other.

I've quoted this section from Ben Lowe at Veloviewer.com but if you click on the links further down and have a good read you'll realise how inaccurate segment times/lengths can be.

The best you can do is make sure you are recording GPS data every second. Some basic Garmins only have what they call Smart recording (records GPS position at anywhere up to about 10 sec intervals depending on speed and any change of direction). The more sophisticated Garmins let you choose between Smart Recording and 1 Sec recording.

Why does this happen?


Strava’s current segment matching algorithm (at the time of writing) uses the closest recorded gps points to the segment’s start and finish points as the start and end points for your effort. These points have to be within a certain distance for the segment matching to be triggered which is why, if your gps device was struggling with accuracy as you pass the start or finish of a segment, it might not show that you completed the segment.

Lets say there is a 20m margin of error for these start and finish points to try and account for gps recording inaccuracies then there is a potential for a segment to be matched but using the gps points 20m short of both start and finish points resulting in a 40m shorter distance. It could of course use a point recorded the other side of the start finish so potentially resulting in 40m longer distance. Comparing those two efforts could result in a 80m discrepancy between two riders who could well have been riding next to each other at the time.

Gps devices themselves contribute to this by having the option to not always record a point every second. In order to save memory, some gps devices give the option of not recording points when they add no extra detail, e.g. if riding at a steady speed on a straight road then you could record points every 5 seconds and still be just as accurate. Now if that is the case then potentially only 1 gps point was recorded in the 20m radius around a segment start/finish point.

https://blog.veloviewer.com/41mph-the-evidence-against-the-sunday-times-article/

then leads on to this article as well

https://blog.veloviewer.com/alternative-leaderboard/

2) Someone has lost their KOM on a segment they created and have now extended it round the corner as they kept the effort up a bit longer and have now 'reclaimed' it.
 
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