I'm a CIQ developer and recently I get more and more user reports and reviews (from different devices) complaining about some datafields, arguing that the DF is crap because they had a 10km hike and the activity only shows way less. I usually dismissed these, as it's not possible for my CIQ datafield to cause that, because the distance, elevation, time are recorded by the built-in activity, my datafield mostly displays things and adds some custom fields.
However yesterday I participated in an open water swimming event, and it kind of happened to me as well. Except that in the open water swimming activity I don't have any CIQ datafields. Now I have to admit that this was my 2nd time using open water swim, the 1st one was a few weeks ago but there I have no way to evaluate whether the measured distance is close to reality. But yesterday's event was an official 1500m swim. My fr965 measured it as 850m. It can be argued whether I should've started, stopped the watch at the time my foot entered/exited the water, or was I right to only measure where I actually swam, but surely that can't be more than 100m difference at the beginning, and 100 m at the end. The recorded activity is still way too short. What's even interesting is that the GPS endpoint of my swim is kind of in the middle of the course, and not opposite to the beach where we ended.
I knew that GPS is not very accurate underwater, so I sometimes raised my hand above the water level for a few seconds (hoping the GPS will get a fix and make the course more accurate in the recording) at least 8-10 times on the way, and I also looked at my watch from time to time, so I know for sure it was recording all the time (it didn't pause or stop). However both compared to the official buoys and my guess based on time (which I can only compare to my swimming pool pace) I noticed pretty much at the beginning that the watch measures significantly less than the distance between the buoys tells.
So I started to believe that there might be one or even 2 bugs in the fr965 related to how it measures distance.
1. maybe open-water swim has a problem with GPS accuracy, much worse than I expected
2. it looks like the recorded course can be way shorter than it should be (meaning the end point is not where it should be, as if I stopped mid-way)
The red X is where we finished