Related
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Open water swim distance calculated by watch vs GPS distance recorded by watch

jonathancolledge
over 5 years ago

What are the reasons for the difference in open water swim distance calculated by watch vs GPS distance recorded by watch? Sometimes one is greater than the other, and other times it is the other way round.

This is a curiosity thing by the way, you only see differences if you analyse the fit file using a tool such as fitfilerepair. 

  • 0 over 5 years ago

    The reason would be that GPS in open water swimming is problematic, because the watch spends most of the time submerged, where no GPS signal is available. The watch gets GPS signal only intermittently, and for very short periods of time. This causes severe accuracy problems. So for open water swimming the watch's software applies special algorithms to recalculate distance covered to remedy this.

  • 0 over 5 years ago in reply to tmk2

    Do you have any more details on these algorithms? I figured this when I finish a swim with my 910XT, if it doesn't have a GPS fix, the distance is extrapolated and so is greater than the GPS measured distance.That seems easy and sensible. My 945 always records GPS position in it's records every 1 second, of course it won't have signal all that time, so why put in the coordinates and how did it know it was there without the GPS fix? A 945 swim will have 1000s of GPS coordinates vs a 910 XT with just 22! How does it calculate the GPS position for each record and how do I know which records are the ones with propper GPS fix? I guess if it sees a zig-zaggy GPS line, at some point it decides this is not your actual swim distance and you likely went straighter, but how does it know if it is only getting a fix a similar number of times to the 910XT? Also, in what situation does it increase your distance apart from the first 910XT example? If it sees a very acute turn that isn't 180 degrees due to infrequent GPS fix, does it assume a longer turn? Sorry, a bit all over the place in my description.

  • 0 over 5 years ago in reply to jonathancolledge
    jonathancolledge said:
    Do you have any more details on these algorithms?

    Sorry, no.

    Besides, I think different models have different algorihtms. The newer watches might work differently due to different GPS chips used.