GPS Open Water error

Loved to swim at Cascais
https://connect.garmin.com/modern/activity/2830546828
This was a swim to a restaurant :-)

Now I have problems with my Fenix 5 on OWS.
I may switch to the SWIM 2.

 tracking route is accurate but distance was reported as 3196 when actual distance should have been around 1800.  This was with glonass. 

If you see it closer, you can find lots of long straight lines in the route, showing that you had lots of GPS dropouts. A correct Swim2 track is much much smoother. If you check the pace chart, you will see lots of constant pace, coming from accelerometer data.

This approach from Garmin to "hide" GPS dropouts by inserting accelerometer data is a real disaster. First it causes massive overreadings, and also it hides the GPS dropout during swim (I can not recognize it by having a simple distance alert)

From my experience GPS ONLY is the way to go. With +GLONASS you have more satellites, but Garmin seems to need more time to figure out how to handle that extra information properly. More data, but less accurate. If you check some running forums, many people report that GPS only is the most accurate way with Garmin watches.

I was having a similar problem with OW swim distance reading  about 25% above what was was expected. My wife also has a Swim 2 and was swimming parallel to me but getting correct GPS and distance readings. I attached my Swim 2 to a swim buoy so it wouldn't lose GPS contact and pulled it behind me -it matched her readings so I knew the watch was capable of accurate GPS readings but was losing signal when I swam.. I switched watches with her and I got the exaggerated distances on her watch - she got accurate reading swimming with my watch. GRRR... So I figured it might be my swim stroke (BTW I swim freestyle) Although I rolled my eyes at Garmin's suggestion to switch the watch to my other wrist - when I moved my watch from my left to right wrist...the problem disappeared. Sounds crazy but it worked. 

Actually my experience shows the same. On left wrist bad readings and steady GPS loss, on right wrist mostly correct readings.

Today I tried with pool swim the same thing (switching from left to right), and the stroke recognition started to work much much better.

Reading this forum but i have a Fenix 6X Pro with the same issues you guys are discussing here. Much longer distance on the OW swims, and btw, also stroke recognition issues in the pool,  Picking up some info here I will try GPS only (without Glonass, etc) and what you say, try it on my right wrist?  That seems crazy.  Does it matter if I swim E to W and then back W to E?  Do I have to change wrist side depending on direction of swim?  Seems nuts.  I think there are two issues.

1.  The accelorameter and GPS corrections being added together to give a much longer distance.

2.  Why is GPS lost for most of the swim to begin with.  

It is a serious suggestion to try it on the other wrist. For me both OW and pool the watch works brilliantly on the right wrist, but is a disaster on my left wrist. You should not change it according to the direction, just it seems that the watch works better on one wrist.

- On my right wrist the GPS signal is always kept, if accidentally lost, the watch finds it really fast during swim, and usually no issues at all. Now with the newest fix even if there is a longer, like several minutes long GPS loss, the distance seems to be correct. In the pool stroke detection is mostly correct.

- on my left wrist in OW no matter which GPS combination I use, GPS signal is lost immediately, and found just occasionally for 1-2 sec only. At least with the fix the distance seems to be ok, but I checked it only with short distances, as I do not want to lose the map track. In the pool freestyle is always misdetected as breaststroke.

All of that symptoms with a symmetrical freestyle stroke (at least what I believe). But even if not symmetrical, left and right wrists are about equal time over the water surface, so keeping the GPS signal should not be a problem on left wrist.

Note:

With Swim2 you can not configure the used arm, it is hardwired as left arm (in settings.fit file). With Fenix 6X Pro you can try to change this configuration, maybe it helps. I would be happy if it would help, as in that case we could ask Garmin to add this configuration. As I wear my watch normally on my left wrist, it is a (minor) hassle to change it always before swim.

On my 6X my wrist settings in for LEFT which is the side that I wear it on.  During one of my experiments I wore the 6X on the right hand, and you are right, looking back the GPS track is better, distance was too long though. I think until the SW update makes it to the fenix 6 I can try GPS only and worn on right hand.  So now if you pause with good signal the GPS doesn't add distance?

Actually I can not answer it for sure, as on my right hand the signal was always good recently. However on left hand with lots of GPS signal loss the distance is not doubled any more. So I say, yes, the fix solves the overblown distance problem.

Trying to control the variables so today I swam with my 6x in GPS only.  In this mode I saw no difference in the track, pretty much straight lines and the distance is still off.  The distance off should be resolved with a similar SW fix that was done on the SWIM2 but the fact that GPS signal is hardly there is still an issue.  Next swim I will wear my 6X on my right hand although I struggle to understand how that can make a difference, other than maybe if one hand stays out of the water longer, etc.