Open water swimming

Why you track it as a "walk"? I always use safety buoy when I swim in open water. Attach my FR945 at one of the outer handle bars and start activity as open water swim.
Distance is tracked fine - HR data I sync via my HRM-Tri chest. Only thing that is of course missing are swim metrics like stroke count.
But like this there are no manual changes in Garmin Connect needed.

It is useful to do a GPS "soak" before pushing start and starting the open water swim for at least a minute after it establishes a GPS fixed. Ideally more. Similarly if you do an out and back swim, it is more accurate if you tread water with your hand above water for a short bit. At the end also as it uses and algorithm to estimate distance since it is under water much of the time so stopping reduces estimation. Most reviews suggest that if it gets within 10% of true distance (measured on a buoy or in a cap as some here do) the watch is doing pretty well. My experience has been close to spot on doing the above.

We’ve not been  able to swim for a few weeks but here’s one of the swims from my wife’s 945 - https://connect.garmin.com/modern/activity/4598205743

The course is out and back along a buoy line with buoys at 250m intervals. it’s approximately 2000m out and back. This is not an exception, it’s the norm; 945 on open water swim setting on wrist swimming freestyle. 

With swimming there is always the possibility that when stopped the watch will seek to establish a ‘true’ position rather than the estimated position it has while swimming. Hence, if you stop and lift your watch clear of the water it will likely ‘catch up’ and display what you perceive to be drift. If you give it a few minutes it’ll settle down and just display the usual drift due to precession.

Overall, we were very happy with the 945 in open water swimming during the summer. Just a pity we lost the last month as swimming was deemed a risky activity here due to the COVID-19 lockdown in New Zealand.

Thanks.

It seems that I have a problem with my Swim2 watch, my "drift" is way more than what can be seen as a simple drift. Drifting 600m in 2 minutes is ridiculous. What I usually see is that people are happy with their Swim2 OW capabilities.

I contacted Garmin Support.

Unless is there is an issue with your watch a lot of it will depend on stroke and stroke technique. Freestyle with a reasonable technique is the best way to get a good result. If you’re seeing 600m+ correction when you’re stopped then it’s not drift, it’s the watch having lost the signal, found it and correcting.

Hey, You hit the nail on the head!! - thanks a lot!

I still do not get, why my freestyle technique is not reasonable. My old FR910XT can handle it easily. I have a slow stroke rate, but beside of that the watch should be able to handle it.

If I sum all the "drifts", it gives the proper length of the swim! And yes, all of the drift distance makes sense.

But there is a big question here, how does the watch measure distance while not having the GPS signal?? I guess the accelerometers come into the picture, but I can not understand why are they not synchronised

In my case I have a guess measurements according to the accelerometers, and a proper catch up distance from the GPS signal. That adds up, resulting a rough double distance always. How can I switch off the accelerometer guesswork?

More questions went to support.

Yesterday I swam 2400 m, did the GPS soak on beforehand and afterwards. Thought it was strange that as I was swimming out it read 1014 m at the turn point but 2440 m when I returned. And the speed! Just faster and faster! So I checked with a walk along the shore with the same GFR945 and it measured 700m each way, totalling 1400m. A margin of error of >1000m! What is wrong? 

Watch was not able to keep the GPS signal. It guessed from accelerometer data - check pace chart, you will see long periods of constant pace, something like 2:35 or 1:46 sec per 100m. And you will see sudden pace increases, when the watch found the GPS signal again. Unfortunately the watch adds all of these measurements, so both the accelerometer guesswork and the measured distance from GPS. This makes a total distance about double the real distance.

You either used wrong technique, or just hit the software bug/inability of the watch to recover fast from GPS signal loss during swim.

If you continue to experience this issue, report to Garmin support. They do know about this problem, but the more people report it, the more resources they will add to sort this out.

Thank you for the answer. It makes perfect sense. Is this an error of this particular unit or do all FR945 units behave like this? I swim mostly freestyle, and my technique is definitely below par. Still I find it odd. I was out swimming again today; same thing happened. Distance 2720 m, but about 1800 m in reality, At Garmin support they suggested to change refresh rate from "smart" to "every second". This did not help. Indoor swimming works fine, though time and again the unit registers an extra lap. Mostly I use swimming as restitution training so I am first and foremost interested in heart rate and time. But this is bugging me anyway. 

5066063 said:

all FR945 units behave like this? I

In fact all watches used in the open water behave like this to a greater or lesser extent. And that usually comes down to swim stroke. If you go back to the very first watches Garmin developed for OWS (310XT I believe) the results were truly horrible as they worked at refining the ‘magic’ that guesstimated the swim from what GPS signals it can detect. That’s why sometimes after an update it will get bad again.

If you’re regularly gaining a lap in the pool then again, it’s likely something you’re doing at the end of a set or perhaps during the swim. 

Of course, you could have a defective watch in which case you should contact Garmin Support.