Interesting stuff. You may also want to try the Log Viewer, which lets you visualise tlog files directly. I don’t believe it supports filtering the displayed path by values within the logviewer though, so if you wanted to test that you’d need to do something like process the tlog (e.g. with pymavlink) to remove the data you wanted to filter out and then analyse/view the result
Your recorded data is quite a bit more precise than the display shows while you’re operating. QGC rounds to the nearest 10cm for the display, which I assume is to avoid the number taking up unnecessary space, especially since when you’re operating a vehicle over a scale of 10s to 100s of meters vertically, showing more precision than 0.1m is generally unhelpful.
Whether the longer one is necessary would depend on the size of the swells, as well as where the antenna is placed on the ROV. Given the focus of an ROV is generally the diving part, it’s probably best to stick with the shorter antenna unless the longer one is absolutely required
