Open Water Swim performance

yeah, the 945LTE Garmin Connect swim activity says I swam 2,269 yards but when I download that activity course to a GPX course file and analyze it the course is around 3,000 yards and fairly accurate to what I swam (not quite as accurate as the course recorded by 735XT but decent).  The difference seems to be when the 945LTE doesn't have GPS it seems to think I'm not moving and I magically jump to the next location, without actually swimming it).  Very odd.  On my runs, the 945LTE produced course is practically within +/- 1 meter uncanny accuracy so the GPS chip seems to be impressive in ideal conditions.

Not sure how you *downloaded gpx to analyze it" and get the right distance.  However i did notice that strava on the pc has a distance correction button (much like garmin has the elevation correction button). When i clicked the strava distance auto correct the distance went from 3333 yards to a much more accurate 3950 yards. At a min Garmin should do the same and have the ability to post swim auto correct distance

interesting. I will turn on sync to Strava and try.  on web-based Garmin Connect if you click the gear icon in the upper right it gives the option to download your activity to a variety of file formats.  One of them is a GPX file which is just a collection of the GPS data. There are a number of tools on the web to analyze the course which gives distance, elevation, etc

I’d guess Garmin is trying to filter the gps data perhaps in conjunction with wrist accelerometer data in SW to guess at real distance (with assumption that GPS would have errors or bad readings).  It sounds like in your case their algorithm is not working well and your gps data may be more accurate.

Looking at fit file data the distance recorded and accumulated doesn’t directly line up with the gps points so clearly something is going on (for me, however the differences haven’t been that different with either number usually being believable).

do you find your moving time to be accurate vs your total time and elapsed time? The issue seems to be that my 945LTE thinks I'm standing still for 12 minutes and not counting the distance swam during that time.  My 3 year old 735XT is infinitely more accurate which is frustrating.  I would think my accelerometer is broken but when I flick my wrist to see the time the  backlight comes on, which I believe would be the same accelerometer!

that's pretty good! even my crazy accurate 735XT regularly shows a 2-3 minute moving time gap.  

i believe that the minutes that garmin shows (wrongly) as not moving are the minutes, through out your swim that the watch didn't have GPS signal. Strava is able to post process that data and give you a more accurate distance. 

in my case it was about 13 minutes which is about 600 yards that on avg i would swim in 13 min, and that's roughly the distance i was short

that is definitely what it is doing, which is absurd, especially in open water where most people rarely stop and GPS is likely to be lost many times. seems pretty simply that once it regains GPS you swam that distance and that you didn't teleport or take a boat!  I waste so much time troubleshooting Garmin products over the years, it might be time to try something else.

If you look at the post i put in on the fenix forum I try to explain what I see going on by just viewing the fit files.  But essentially, its sort of like a roller coaster of the gps chip seeing GPS then loosing it.  It can have GPS for 5 min, then lose it for 30 seconds, and this can repeat many times depending on your swim style, duration, etc.  Every time it regains GPS signal it sort of ramps up to the right speed.  This ramp up can take 30 seconds, so for those 30 seconds you lose some recorded distance.  The more this cycle (loosing and regaining gps signal) is repeated, the larger the error.  Hopefully there is something they can do about that but if not, at least emulate what Strava is doing by post processing the file and calculate distance based on the track itself.  Strava does this fairly accurately.  Who knows but they (Garming) need to get back to basics and make sure that Swim, Run and Bike work 100%.  What is annoying to me is that all prior watches I had from Garmin I never had a swim issue.  They always seem to take three steps forward and one back.  When you buy a their newest and latest watch, you dont expect it to work worse (in some areas) then the older versions you retired.  Love, Hate.